diff --git a/STATUS.md b/STATUS.md
index 6d38d0e..ffb8a25 100644
--- a/STATUS.md
+++ b/STATUS.md
@@ -98,7 +98,7 @@ Vollständiger Plan: [`mana/docs/playbooks/CARDS_GREENFIELD.md`](../mana/docs/pl
 | 9 | Polish (DSGVO-UI, Settings, Account, Statistik, i18n, A11y, Media, Image-Occlusion) | 🟡 weit | Card-Edit + Cloze-Editor + Inbox-Banner + Account/DSGVO + Statistik + Pre-Flight-Swap + i18n DE/EN + A11y-Pass + Cloze-Hint-Anzeige + Anki-Re-Import-Dedupe + MinIO-Media-Upload + Image-Occlusion durch (9a–9l). Verbleibend: type-in, audio, multiple-choice (Schema vorbereitet) |
 | 10 | Production-Deploy (Mac Mini, Cloudflare-Tunnel) | ✅ live 2026-05-08 | cardecky.mana.how + cardecky-api.mana.how, alte cards.* via nginx-301-Redirect |
 | 11 | Decommission Cards-Modul aus mana-monorepo | ✅ 2026-05-08 | apps/cards, services/cards-server, packages/cards-core, mana-app cards-Modul + cross-refs entfernt (4 Commits, type-check 0 errors) |
-| 12 | Marketplace-Restore (R0–R6) | 🟡 R0+R1+R2 durch | Plan: [`docs/playbooks/MARKETPLACE_RESTORE.md`](docs/playbooks/MARKETPLACE_RESTORE.md). R0 (Doku-Archiv + Restore-Plan + Strategie-B-Klarstellung): ✅. R1 (16 Tabellen + 5 Enums in `marketplace`-pgSchema, CHECK-Constraint verifiziert): ✅. R2 (Backend α + β): ✅ — Author-Routen (`POST/GET /authors/me`, `GET /authors/:slug`), Deck-Init (`POST /decks`), Publish-Flow (`POST /decks/:slug/publish` mit @cards/domain-Hash + per-Version-Hash + AI-Mod-Stub-Log + atomarem latest_version_id-Bump), PATCH-Metadaten, Public-Detail mit optional-auth. 16 neue Tests (72 gesamt) grün, E2E-Smoke gegen lokale cards-api durch (Cardecky-Author + Deck `r2-stoische-ethik` mit 3 Karten v1.0.0, semver-409 + paid-422-Errors verifiziert). Code-Referenz aus altem cards-server unter `docs/marketplace/archive/code/` (3331 LOC). Verbleibend: R3 γ/δ (Discovery + Subscribe + Smart-Merge), R4 ε (PRs + Discussions), R5 Frontend-Routes, R6 voller E2E-Smoke. |
+| 12 | Marketplace-Restore (R0–R6) | 🟡 R0+R1+R2+R3 durch | Plan: [`docs/playbooks/MARKETPLACE_RESTORE.md`](docs/playbooks/MARKETPLACE_RESTORE.md). R0 (Doku + Restore-Plan + Strategie-B-Klarstellung): ✅. R1 (16 Tabellen + 5 Enums in `marketplace`-pgSchema): ✅. R2 (Backend α + β: Author-Routen + Deck-Init + Publish-Flow): ✅. **R3 (γ + δ Discovery + Engagement + Subscribe + Smart-Merge): ✅** — `GET /explore` (featured + trending), `GET /decks` (browse mit q/tag/lang/author/sort/limit/offset), `GET /tags`, `POST/DELETE/GET /decks/:slug/star`, `POST/DELETE/GET /authors/:slug/follow`, `POST/DELETE/GET /decks/:slug/subscribe`, `GET /me/subscriptions` (mit update_available-Flag), `GET /decks/:slug/versions/:semver`, `GET /decks/:slug/diff?from=:semver`, `POST /decks/:slug/fork` (private cards.decks-Kopie mit `forked_from_marketplace_*`-Pointern + frischen FSRS-Reviews), `POST /private/:deckId/pull-update` (Smart-Merge-Pull: hash-equality dedupe via `@cards/domain.cardContentHash` lässt unveränderte Karten **inkl. ihrer FSRS-Reviews komplett in Ruhe**, neue/geänderte Karten kommen als private Insert dazu). 6 neue Diff-Heuristik-Unit-Tests, 78 gesamt grün. **End-to-End-Smoke verifiziert**: Cardecky publisht v1.0.0 → Till forkt → Till studiert Apatheia (state=review, stability=10, reps=3) → Cardecky publisht v1.1.0 (Logos geändert + Tugendlehre neu) → Till pull-update → Apatheia-Review intakt, +Tugendlehre + neue Logos-Karte als zusätzliche Inserts. Verbleibend: R4 ε (PRs + Discussions), R5 Frontend-Routes, R6 voller End-to-End-Smoke gegen UI. |
 
 Legende: ✅ erledigt + verifiziert · 🚧 blockiert · ⏸ noch nicht begonnen
 
diff --git a/apps/api/src/db/schema/decks.ts b/apps/api/src/db/schema/decks.ts
index 06e65e4..71045da 100644
--- a/apps/api/src/db/schema/decks.ts
+++ b/apps/api/src/db/schema/decks.ts
@@ -21,6 +21,14 @@ export const decks = cardsSchema.table(
 			.default('private'),
 		fsrsSettings: jsonb('fsrs_settings').notNull().default(sql`'{}'::jsonb`),
 		contentHash: text('content_hash'),
+		// Marketplace-Lineage (Phase 12 R3): wenn dieses private Deck via
+		// `POST /api/v1/marketplace/decks/:slug/fork` aus einem
+		// öffentlichen Deck entstanden ist, zeigen diese Pointer auf
+		// die marketplace-Quelle. NULL = nicht-geforkt (einfach selbst
+		// angelegt). Wird beim Smart-Merge-Pull benutzt um die richtige
+		// Quelle nachzuladen.
+		forkedFromMarketplaceDeckId: text('forked_from_marketplace_deck_id'),
+		forkedFromMarketplaceVersionId: text('forked_from_marketplace_version_id'),
 		createdAt: timestamp('created_at', { withTimezone: true, mode: 'date' })
 			.notNull()
 			.defaultNow(),
diff --git a/apps/api/src/index.ts b/apps/api/src/index.ts
index 551fb67..3c20b13 100644
--- a/apps/api/src/index.ts
+++ b/apps/api/src/index.ts
@@ -15,6 +15,10 @@ import { mediaRouter } from './routes/media.ts';
 import { decksGenerateRouter } from './routes/decks-generate.ts';
 import { authorsRouter as marketplaceAuthorsRouter } from './routes/marketplace/authors.ts';
 import { marketplaceDecksRouter } from './routes/marketplace/decks.ts';
+import { exploreRouter as marketplaceExploreRouter } from './routes/marketplace/explore.ts';
+import { engagementRouter as marketplaceEngagementRouter } from './routes/marketplace/engagement.ts';
+import { subscriptionsRouter as marketplaceSubscriptionsRouter } from './routes/marketplace/subscriptions.ts';
+import { forkRouter as marketplaceForkRouter } from './routes/marketplace/fork.ts';
 
 const app = new Hono();
 
@@ -50,6 +54,14 @@ app.route('/api/v1/decks/generate', decksGenerateRouter());
 
 // Marketplace (Phase 12). Eigenes pgSchema, additive Routen unter /v1/marketplace/*.
 // Plan: docs/playbooks/MARKETPLACE_RESTORE.md.
+//
+// Mount-Reihenfolge ist signifikant: spezifischere Routes vor /authors
+// und /decks, damit z.B. /marketplace/me/subscriptions nicht zu authors
+// oder decks geroutet wird.
+app.route('/api/v1/marketplace', marketplaceExploreRouter());
+app.route('/api/v1/marketplace', marketplaceEngagementRouter());
+app.route('/api/v1/marketplace', marketplaceSubscriptionsRouter());
+app.route('/api/v1/marketplace', marketplaceForkRouter());
 app.route('/api/v1/marketplace/authors', marketplaceAuthorsRouter());
 app.route('/api/v1/marketplace/decks', marketplaceDecksRouter());
 
diff --git a/apps/api/src/lib/marketplace/diff.ts b/apps/api/src/lib/marketplace/diff.ts
new file mode 100644
index 0000000..6aadeef
--- /dev/null
+++ b/apps/api/src/lib/marketplace/diff.ts
@@ -0,0 +1,89 @@
+/**
+ * Smart-Merge-Diff zwischen zwei Versionen oder zwischen einer Version
+ * und der Hash-Liste eines privaten geforkten Decks.
+ *
+ * Klassifikation:
+ *   - **unchanged**: content_hash identisch zwischen alt und neu →
+ *     unveränderte Karte, FSRS-State bleibt.
+ *   - **added**: Hash nur in neu → komplett neue Karte.
+ *   - **removed**: Hash nur in alt → Karte aus dem Deck entfernt.
+ *   - **changed** (Heuristik): Karte am selben `ord` ist „added" und
+ *     gleichzeitig an demselben `ord` eine „removed". Pull-Requests
+ *     bringen später eine echte Karten-Lineage; bis dahin reicht die
+ *     Position-Heuristik.
+ *
+ * Geport aus
+ * `cards-decommission-base:services/cards-server/src/services/subscriptions.ts`
+ * (`diffSince`-Funktion, ord-Pairing-Heuristik 1:1).
+ */
+
+export interface DiffCardForHash {
+	contentHash: string;
+	ord: number;
+}
+
+export interface DiffCardFull {
+	contentHash: string;
+	type: string;
+	fields: Record<string, string>;
+	ord: number;
+}
+
+export interface DiffPayload {
+	from: { semver?: string; versionId?: string };
+	to: { semver: string; versionId: string };
+	added: DiffCardFull[];
+	changed: { previous: { contentHash: string }; next: DiffCardFull }[];
+	unchanged: { contentHash: string; ord: number }[];
+	removed: { contentHash: string }[];
+}
+
+export interface ComputeDiffInput {
+	from: DiffCardForHash[];
+	to: DiffCardFull[];
+	fromInfo: { semver?: string; versionId?: string };
+	toInfo: { semver: string; versionId: string };
+}
+
+export function computeDiff(input: ComputeDiffInput): DiffPayload {
+	const { from, to, fromInfo, toInfo } = input;
+	const fromHashes = new Set(from.map((c) => c.contentHash));
+	const toHashes = new Set(to.map((c) => c.contentHash));
+
+	const unchanged: { contentHash: string; ord: number }[] = [];
+
+	// Build ord → hash map of removed cards (in `from` but not in `to`).
+	const removedByOrd = new Map<number, string>();
+	for (const c of from) {
+		if (!toHashes.has(c.contentHash)) removedByOrd.set(c.ord, c.contentHash);
+	}
+
+	const added: DiffCardFull[] = [];
+	const changed: { previous: { contentHash: string }; next: DiffCardFull }[] = [];
+
+	for (const c of to) {
+		if (fromHashes.has(c.contentHash)) {
+			unchanged.push({ contentHash: c.contentHash, ord: c.ord });
+		} else if (removedByOrd.has(c.ord)) {
+			const previousHash = removedByOrd.get(c.ord)!;
+			removedByOrd.delete(c.ord);
+			changed.push({ previous: { contentHash: previousHash }, next: c });
+		} else {
+			added.push(c);
+		}
+	}
+
+	// Was in removedByOrd übrig bleibt = echte Removals (nicht ord-paired).
+	const removed: { contentHash: string }[] = [...removedByOrd.values()].map((h) => ({
+		contentHash: h,
+	}));
+
+	return {
+		from: fromInfo,
+		to: toInfo,
+		added,
+		changed,
+		unchanged,
+		removed,
+	};
+}
diff --git a/apps/api/src/routes/marketplace/decks.ts b/apps/api/src/routes/marketplace/decks.ts
index 9e90d4e..6659301 100644
--- a/apps/api/src/routes/marketplace/decks.ts
+++ b/apps/api/src/routes/marketplace/decks.ts
@@ -156,12 +156,12 @@ export function marketplaceDecksRouter(
 		});
 	});
 
-	// Authenticated endpoints folgen.
-	const auth = new Hono<{ Variables: AuthVars }>();
-	auth.use('*', authMiddleware);
+	// Authenticated endpoints — per-route authMiddleware statt Sub-
+	// Router-Mount, damit das Wildcard nicht die Public-GET-Routes
+	// fängt.
 
 	// POST / — Deck-Init.
-	auth.post('/', async (c) => {
+	r.post('/', authMiddleware, async (c) => {
 		const userId = c.get('userId');
 		const body = await c.req.json().catch(() => null);
 		const parsed = InitSchema.safeParse(body);
@@ -226,7 +226,7 @@ export function marketplaceDecksRouter(
 	});
 
 	// PATCH /:slug — Metadaten.
-	auth.patch('/:slug', async (c) => {
+	r.patch('/:slug', authMiddleware, async (c) => {
 		const userId = c.get('userId');
 		const slug = c.req.param('slug');
 		const body = await c.req.json().catch(() => null);
@@ -265,7 +265,7 @@ export function marketplaceDecksRouter(
 	});
 
 	// POST /:slug/publish — neue Version.
-	auth.post('/:slug/publish', async (c) => {
+	r.post('/:slug/publish', authMiddleware, async (c) => {
 		const userId = c.get('userId');
 		const slug = c.req.param('slug');
 		const body = await c.req.json().catch(() => null);
@@ -372,9 +372,5 @@ export function marketplaceDecksRouter(
 		);
 	});
 
-	// Auth-Sub-Router gemountet auf '/'. Hono routet zuerst exakte
-	// Pfade auf `r` (GET /:slug), Rest fließt auf den auth-Mount.
-	r.route('/', auth);
-
 	return r;
 }
diff --git a/apps/api/src/routes/marketplace/engagement.ts b/apps/api/src/routes/marketplace/engagement.ts
new file mode 100644
index 0000000..821c629
--- /dev/null
+++ b/apps/api/src/routes/marketplace/engagement.ts
@@ -0,0 +1,130 @@
+import { and, eq } from 'drizzle-orm';
+import { Hono } from 'hono';
+
+import { getDb, type CardsDb } from '../../db/connection.ts';
+import { authorFollows, authors, deckStars, publicDecks } from '../../db/schema/index.ts';
+import { authMiddleware, type AuthVars } from '../../middleware/auth.ts';
+import { optionalAuthMiddleware } from '../../middleware/marketplace/optional-auth.ts';
+
+/**
+ * Engagement-Primitives — Stars (Bookmarks für Decks) + Follows
+ * (Folgen von Authoren).
+ *
+ * Stars sind idempotent: doppeltes POST = no-op.
+ * Follows sind idempotent: Self-Follow wird mit 409 abgelehnt.
+ *
+ * State-Endpoints (`GET /:slug/star`, `GET /:slug/follow`) sind
+ * optional-auth: anonym = false, signed-in = wahre Antwort.
+ */
+
+export type MarketplaceEngagementDeps = { db?: CardsDb };
+
+async function findDeckBySlug(db: CardsDb, slug: string) {
+	const [row] = await db.select().from(publicDecks).where(eq(publicDecks.slug, slug)).limit(1);
+	return row ?? null;
+}
+
+async function findAuthorBySlug(db: CardsDb, slug: string) {
+	const [row] = await db.select().from(authors).where(eq(authors.slug, slug)).limit(1);
+	return row ?? null;
+}
+
+export function engagementRouter(
+	deps: MarketplaceEngagementDeps = {}
+): Hono<{ Variables: Partial<AuthVars> }> {
+	const r = new Hono<{ Variables: Partial<AuthVars> }>();
+	const dbOf = () => deps.db ?? getDb();
+
+	// ─── Stars (Decks) ───────────────────────────────────────────────
+	// Per-route authMiddleware statt Sub-Router-Mount, damit die
+	// Public-State-Routes (`GET /decks/:slug/star` mit optional-auth)
+	// nicht durch ein Wildcard-Middleware gefangen werden.
+
+	r.post('/decks/:slug/star', authMiddleware, async (c) => {
+		const userId = c.get('userId');
+		const slug = c.req.param('slug');
+		const db = dbOf();
+		const deck = await findDeckBySlug(db, slug);
+		if (!deck) return c.json({ error: 'not_found' }, 404);
+		await db.insert(deckStars).values({ userId, deckId: deck.id }).onConflictDoNothing();
+		return c.json({ starred: true });
+	});
+
+	r.delete('/decks/:slug/star', authMiddleware, async (c) => {
+		const userId = c.get('userId');
+		const slug = c.req.param('slug');
+		const db = dbOf();
+		const deck = await findDeckBySlug(db, slug);
+		if (!deck) return c.json({ error: 'not_found' }, 404);
+		await db
+			.delete(deckStars)
+			.where(and(eq(deckStars.userId, userId), eq(deckStars.deckId, deck.id)));
+		return c.json({ starred: false });
+	});
+
+	// GET /decks/:slug/star — own state, optional-auth (anon = false).
+	r.get('/decks/:slug/star', optionalAuthMiddleware, async (c) => {
+		const userId = c.get('userId');
+		if (!userId) return c.json({ starred: false });
+		const slug = c.req.param('slug');
+		const db = dbOf();
+		const rows = await db
+			.select({ id: deckStars.deckId })
+			.from(deckStars)
+			.innerJoin(publicDecks, eq(publicDecks.id, deckStars.deckId))
+			.where(and(eq(deckStars.userId, userId), eq(publicDecks.slug, slug)))
+			.limit(1);
+		return c.json({ starred: rows.length > 0 });
+	});
+
+	// ─── Follows (Authors) ───────────────────────────────────────────
+
+	r.post('/authors/:slug/follow', authMiddleware, async (c) => {
+		const followerUserId = c.get('userId');
+		const slug = c.req.param('slug');
+		const db = dbOf();
+		const author = await findAuthorBySlug(db, slug);
+		if (!author) return c.json({ error: 'not_found' }, 404);
+		if (author.userId === followerUserId) {
+			return c.json({ error: 'cannot_follow_self' }, 409);
+		}
+		await db
+			.insert(authorFollows)
+			.values({ followerUserId, authorUserId: author.userId })
+			.onConflictDoNothing();
+		return c.json({ following: true });
+	});
+
+	r.delete('/authors/:slug/follow', authMiddleware, async (c) => {
+		const followerUserId = c.get('userId');
+		const slug = c.req.param('slug');
+		const db = dbOf();
+		const author = await findAuthorBySlug(db, slug);
+		if (!author) return c.json({ error: 'not_found' }, 404);
+		await db
+			.delete(authorFollows)
+			.where(
+				and(
+					eq(authorFollows.followerUserId, followerUserId),
+					eq(authorFollows.authorUserId, author.userId)
+				)
+			);
+		return c.json({ following: false });
+	});
+
+	r.get('/authors/:slug/follow', optionalAuthMiddleware, async (c) => {
+		const followerUserId = c.get('userId');
+		if (!followerUserId) return c.json({ following: false });
+		const slug = c.req.param('slug');
+		const db = dbOf();
+		const rows = await db
+			.select({ id: authorFollows.authorUserId })
+			.from(authorFollows)
+			.innerJoin(authors, eq(authors.userId, authorFollows.authorUserId))
+			.where(and(eq(authorFollows.followerUserId, followerUserId), eq(authors.slug, slug)))
+			.limit(1);
+		return c.json({ following: rows.length > 0 });
+	});
+
+	return r;
+}
diff --git a/apps/api/src/routes/marketplace/explore.ts b/apps/api/src/routes/marketplace/explore.ts
new file mode 100644
index 0000000..a0927f9
--- /dev/null
+++ b/apps/api/src/routes/marketplace/explore.ts
@@ -0,0 +1,225 @@
+import { and, asc, count, desc, eq, ilike, or, sql } from 'drizzle-orm';
+import { Hono } from 'hono';
+import { z } from 'zod';
+
+import { getDb, type CardsDb } from '../../db/connection.ts';
+import {
+	authors,
+	deckTags,
+	publicDeckVersions,
+	publicDecks,
+	tagDefinitions,
+} from '../../db/schema/index.ts';
+import type { AuthVars } from '../../middleware/auth.ts';
+import { optionalAuthMiddleware } from '../../middleware/marketplace/optional-auth.ts';
+
+/**
+ * Discovery — Browse, Search, Featured, Trending, Tag-Tree.
+ *
+ * Pure read-only, optional-auth (anonymer Browse erlaubt; signed-in
+ * User kriegen später hier nicht zusätzliches, aber das State-Lesen
+ * für Star/Subscribe geht über die per-Deck-Endpoints).
+ *
+ * Search nutzt `ILIKE` über Title + Description — gut genug für
+ * Phase γ. tsvector-Upgrade hängt an Phase ι.
+ *
+ * Trending = Star-Velocity der letzten 7 Tage. Bei kleinem N gambar,
+ * fine sobald Volumen kommt — Algorithmus austauschbar ohne API-
+ * Änderung.
+ *
+ * Geport aus
+ * `cards-decommission-base:services/cards-server/src/services/explore.ts`,
+ * mit FQN-Anpassung von `cards.*` auf `marketplace.*`.
+ */
+
+export type MarketplaceExploreDeps = { db?: CardsDb };
+
+const SortEnum = z.enum(['recent', 'popular', 'trending']);
+
+const BrowseQuerySchema = z.object({
+	q: z.string().max(200).optional(),
+	tag: z.string().max(60).optional(),
+	language: z
+		.string()
+		.regex(/^[a-z]{2}$/)
+		.optional(),
+	author: z.string().max(60).optional(),
+	sort: SortEnum.optional(),
+	limit: z.coerce.number().int().min(1).max(100).optional(),
+	offset: z.coerce.number().int().min(0).optional(),
+});
+
+interface DeckListEntry {
+	slug: string;
+	title: string;
+	description: string | null;
+	language: string | null;
+	license: string;
+	price_credits: number;
+	card_count: number;
+	star_count: number;
+	subscriber_count: number;
+	is_featured: boolean;
+	created_at: string;
+	owner: {
+		slug: string;
+		display_name: string;
+		verified_mana: boolean;
+		verified_community: boolean;
+	};
+}
+
+async function browseImpl(
+	db: CardsDb,
+	filter: z.infer<typeof BrowseQuerySchema>
+): Promise<{ items: DeckListEntry[]; total: number }> {
+	const limit = filter.limit ?? 20;
+	const offset = filter.offset ?? 0;
+	const sort = filter.sort ?? 'recent';
+
+	const conditions = [eq(publicDecks.isTakedown, false)];
+	if (filter.language) conditions.push(eq(publicDecks.language, filter.language));
+	if (filter.q) {
+		const like = `%${filter.q}%`;
+		const expr = or(ilike(publicDecks.title, like), ilike(publicDecks.description, like));
+		if (expr) conditions.push(expr);
+	}
+	if (filter.author) {
+		conditions.push(
+			eq(
+				publicDecks.ownerUserId,
+				sql<string>`(SELECT user_id FROM marketplace.authors WHERE slug = ${filter.author} LIMIT 1)`
+			)
+		);
+	}
+	if (filter.tag) {
+		conditions.push(
+			sql`EXISTS (SELECT 1 FROM marketplace.deck_tags dt JOIN marketplace.tag_definitions td ON td.id = dt.tag_id WHERE dt.deck_id = ${publicDecks.id} AND td.slug = ${filter.tag})`
+		);
+	}
+
+	const starCount = sql<number>`(SELECT count(*)::int FROM marketplace.deck_stars s WHERE s.deck_id = ${publicDecks.id})`;
+	const subscriberCount = sql<number>`(SELECT count(*)::int FROM marketplace.deck_subscriptions s WHERE s.deck_id = ${publicDecks.id})`;
+	const cardCountExpr = sql<number>`COALESCE((SELECT v.card_count FROM marketplace.deck_versions v WHERE v.id = ${publicDecks.latestVersionId}), 0)`;
+
+	const sortClause =
+		sort === 'popular'
+			? desc(starCount)
+			: sort === 'trending'
+				? desc(
+						sql<number>`(SELECT count(*)::int FROM marketplace.deck_stars s WHERE s.deck_id = ${publicDecks.id} AND s.starred_at >= now() - interval '7 days')`
+					)
+				: desc(publicDecks.createdAt);
+
+	const rows = await db
+		.select({
+			slug: publicDecks.slug,
+			title: publicDecks.title,
+			description: publicDecks.description,
+			language: publicDecks.language,
+			license: publicDecks.license,
+			priceCredits: publicDecks.priceCredits,
+			cardCount: cardCountExpr,
+			starCount,
+			subscriberCount,
+			isFeatured: publicDecks.isFeatured,
+			createdAt: publicDecks.createdAt,
+			ownerSlug: authors.slug,
+			ownerDisplayName: authors.displayName,
+			ownerVerifiedMana: authors.verifiedMana,
+			ownerVerifiedCommunity: authors.verifiedCommunity,
+		})
+		.from(publicDecks)
+		.innerJoin(authors, eq(authors.userId, publicDecks.ownerUserId))
+		.where(and(...conditions))
+		.orderBy(sortClause)
+		.limit(limit)
+		.offset(offset);
+
+	const totalRow = await db
+		.select({ value: count() })
+		.from(publicDecks)
+		.innerJoin(authors, eq(authors.userId, publicDecks.ownerUserId))
+		.where(and(...conditions));
+
+	return {
+		items: rows.map((r) => ({
+			slug: r.slug,
+			title: r.title,
+			description: r.description,
+			language: r.language,
+			license: r.license,
+			price_credits: r.priceCredits,
+			card_count: Number(r.cardCount),
+			star_count: Number(r.starCount),
+			subscriber_count: Number(r.subscriberCount),
+			is_featured: r.isFeatured,
+			created_at: r.createdAt.toISOString(),
+			owner: {
+				slug: r.ownerSlug,
+				display_name: r.ownerDisplayName,
+				verified_mana: r.ownerVerifiedMana,
+				verified_community: r.ownerVerifiedCommunity,
+			},
+		})),
+		total: totalRow[0]?.value ?? 0,
+	};
+}
+
+export function exploreRouter(
+	deps: MarketplaceExploreDeps = {}
+): Hono<{ Variables: Partial<AuthVars> }> {
+	const r = new Hono<{ Variables: Partial<AuthVars> }>();
+	const dbOf = () => deps.db ?? getDb();
+
+	r.use('*', optionalAuthMiddleware);
+
+	// GET /explore — Featured + Trending Side-by-Side.
+	r.get('/explore', async (c) => {
+		const db = dbOf();
+		const [featured, trending] = await Promise.all([
+			browseImpl(db, { sort: 'popular', limit: 8 }).then((r) =>
+				r.items.filter((d) => d.is_featured).slice(0, 8)
+			),
+			browseImpl(db, { sort: 'trending', limit: 8 }),
+		]);
+		return c.json({ featured, trending: trending.items });
+	});
+
+	// GET /decks — Browse mit Filtern + Sortierung + Pagination.
+	r.get('/decks', async (c) => {
+		const parsed = BrowseQuerySchema.safeParse(Object.fromEntries(new URL(c.req.url).searchParams));
+		if (!parsed.success) {
+			return c.json(
+				{ error: 'invalid_query', issues: parsed.error.issues.map((i) => i.message) },
+				422
+			);
+		}
+		const result = await browseImpl(dbOf(), parsed.data);
+		return c.json(result);
+	});
+
+	// GET /tags — flacher Tag-Tree.
+	r.get('/tags', async (c) => {
+		const rows = await dbOf()
+			.select()
+			.from(tagDefinitions)
+			.orderBy(asc(tagDefinitions.name));
+		return c.json({
+			tags: rows.map((t) => ({
+				id: t.id,
+				slug: t.slug,
+				name: t.name,
+				parent_id: t.parentId,
+				description: t.description,
+				curated: t.curated,
+			})),
+		});
+	});
+
+	// Imports kept alive für künftige Routes (curated-tags, by-version-…).
+	void publicDeckVersions;
+	void deckTags;
+
+	return r;
+}
diff --git a/apps/api/src/routes/marketplace/fork.ts b/apps/api/src/routes/marketplace/fork.ts
new file mode 100644
index 0000000..93875f2
--- /dev/null
+++ b/apps/api/src/routes/marketplace/fork.ts
@@ -0,0 +1,343 @@
+import { and, asc, eq } from 'drizzle-orm';
+import { Hono } from 'hono';
+import { z } from 'zod';
+
+import {
+	cardContentHash,
+	newReview,
+	subIndexCount,
+	subIndexCountForCloze,
+} from '@cards/domain';
+
+import { getDb, type CardsDb } from '../../db/connection.ts';
+import { cards, decks, reviews } from '../../db/schema/index.ts';
+import {
+	publicDeckCards,
+	publicDeckVersions,
+	publicDecks,
+} from '../../db/schema/index.ts';
+import { authMiddleware, type AuthVars } from '../../middleware/auth.ts';
+import { ulid } from '../../lib/ulid.ts';
+import { computeDiff } from '../../lib/marketplace/diff.ts';
+
+/**
+ * Fork + Smart-Merge-Pull-Update.
+ *
+ * **Fork**: kopiert ein Marketplace-Deck (latest published Version) in
+ * eine eigene private `cards.decks`-Row + erzeugt private `cards.cards`-
+ * Zeilen aus den `marketplace.deck_cards`. FSRS-Reviews kommen frisch
+ * (newReview pro subIndex). `cards.decks.forked_from_marketplace_deck_id`
+ * + `…version_id` werden gesetzt — das ist der Anker für spätere
+ * Smart-Merge-Pulls.
+ *
+ * **Pull-Update**: gegen einen privaten geforkten Deck. Berechnet Diff
+ * zwischen geforkter Version und aktueller Marketplace-Latest. Neue
+ * Karten werden eingefügt; geänderte Karten kriegen einen neuen Card-
+ * Insert (mit neuem content_hash) — der **alte** Card-Eintrag bleibt
+ * bestehen, damit existierende FSRS-Reviews intakt bleiben. Removed-
+ * Cards bleiben ebenfalls (User behält History; Pull entfernt nichts
+ * Lokales). Im UI werden sie später dezent als „nicht mehr im Original"
+ * markiert (R5).
+ *
+ * Architektur-Begründung:
+ * - **content_hash-basierter Dedupe** ist die Smart-Merge-Magic:
+ *   `cards.cards.content_hash` matcht `marketplace.deck_cards.content_hash`,
+ *   identisch berechnet via `@cards/domain.cardContentHash`.
+ *   Unveränderte Karten = identisches Hash → schon da → Insert
+ *   übersprungen → FSRS bleibt.
+ * - **Removed-Cards bleiben lokal**: anders als das alte Dexie-Sync
+ *   ist hier der server-authoritative Stand: User entscheidet selbst,
+ *   was er löscht. Re-Pull soll keine User-Daten löschen.
+ */
+
+export type MarketplaceForkDeps = { db?: CardsDb };
+
+const ForkBodySchema = z.object({
+	color: z.string().max(20).optional(),
+});
+
+async function findDeckBySlug(db: CardsDb, slug: string) {
+	const [row] = await db.select().from(publicDecks).where(eq(publicDecks.slug, slug)).limit(1);
+	return row ?? null;
+}
+
+async function loadVersionCards(db: CardsDb, versionId: string) {
+	return db
+		.select()
+		.from(publicDeckCards)
+		.where(eq(publicDeckCards.versionId, versionId))
+		.orderBy(asc(publicDeckCards.ord));
+}
+
+function subIndexCountFor(type: string, fields: Record<string, string>): number {
+	if (type === 'cloze') return subIndexCountForCloze(fields.text ?? '');
+	if (type === 'image-occlusion') {
+		// image-occlusion hat dynamische subIndexes via mask_regions —
+		// im Marketplace-Fork bisher nicht unterstützt. Default 1.
+		return 1;
+	}
+	return subIndexCount(type);
+}
+
+function buildInitialReviews(
+	userId: string,
+	cardId: string,
+	count: number,
+	now: Date
+) {
+	return Array.from({ length: count }, (_, subIndex) => {
+		const review = newReview({ userId, cardId, subIndex, now });
+		return {
+			cardId: review.card_id,
+			subIndex: review.sub_index,
+			userId: review.user_id,
+			due: new Date(review.due),
+			stability: review.stability,
+			difficulty: review.difficulty,
+			elapsedDays: review.elapsed_days,
+			scheduledDays: review.scheduled_days,
+			learningSteps: review.learning_steps,
+			reps: review.reps,
+			lapses: review.lapses,
+			state: review.state,
+			lastReview: review.last_review ? new Date(review.last_review) : null,
+		};
+	});
+}
+
+export function forkRouter(deps: MarketplaceForkDeps = {}): Hono<{ Variables: AuthVars }> {
+	const r = new Hono<{ Variables: AuthVars }>();
+	const dbOf = () => deps.db ?? getDb();
+
+	r.use('*', authMiddleware);
+
+	// POST /decks/:slug/fork — Marketplace → privater Deck.
+	r.post('/decks/:slug/fork', async (c) => {
+		const userId = c.get('userId');
+		const slug = c.req.param('slug');
+		const body = await c.req.json().catch(() => ({}));
+		const parsed = ForkBodySchema.safeParse(body);
+		if (!parsed.success) {
+			return c.json(
+				{ error: 'invalid_input', issues: parsed.error.issues.map((i) => i.message) },
+				422
+			);
+		}
+
+		const db = dbOf();
+		const sourceDeck = await findDeckBySlug(db, slug);
+		if (!sourceDeck) return c.json({ error: 'not_found' }, 404);
+		if (sourceDeck.isTakedown) return c.json({ error: 'takedown_active' }, 403);
+		if (!sourceDeck.latestVersionId) return c.json({ error: 'no_published_version' }, 409);
+
+		const sourceCards = await loadVersionCards(db, sourceDeck.latestVersionId);
+		if (sourceCards.length === 0) return c.json({ error: 'empty_version' }, 409);
+
+		const newDeckId = ulid();
+		const now = new Date();
+
+		const newDeck = await db.transaction(async (tx) => {
+			const [deck] = await tx
+				.insert(decks)
+				.values({
+					id: newDeckId,
+					userId,
+					name: sourceDeck.title,
+					description: sourceDeck.description,
+					color: parsed.data.color ?? '#0ea5e9', // sky-500 — Fork-Default
+					visibility: 'private',
+					fsrsSettings: {},
+					forkedFromMarketplaceDeckId: sourceDeck.id,
+					forkedFromMarketplaceVersionId: sourceDeck.latestVersionId,
+					createdAt: now,
+					updatedAt: now,
+				})
+				.returning();
+
+			for (const sourceCard of sourceCards) {
+				const cardId = ulid();
+				await tx.insert(cards).values({
+					id: cardId,
+					deckId: newDeckId,
+					userId,
+					type: sourceCard.type,
+					fields: sourceCard.fields,
+					mediaRefs: [],
+					contentHash: sourceCard.contentHash,
+					createdAt: now,
+					updatedAt: now,
+				});
+
+				const subIndexes = subIndexCountFor(
+					sourceCard.type,
+					sourceCard.fields as Record<string, string>
+				);
+				const initialReviews = buildInitialReviews(userId, cardId, subIndexes, now);
+				if (initialReviews.length > 0) {
+					await tx.insert(reviews).values(initialReviews);
+				}
+			}
+
+			return deck;
+		});
+
+		return c.json(
+			{
+				deck: {
+					id: newDeck.id,
+					name: newDeck.name,
+					description: newDeck.description,
+					color: newDeck.color,
+					forked_from_marketplace_deck_id: newDeck.forkedFromMarketplaceDeckId,
+					forked_from_marketplace_version_id: newDeck.forkedFromMarketplaceVersionId,
+				},
+				cards_created: sourceCards.length,
+			},
+			201
+		);
+	});
+
+	// POST /decks/:private-deck-id/pull-update — Smart-Merge-Pull.
+	r.post('/private/:deckId/pull-update', async (c) => {
+		const userId = c.get('userId');
+		const deckId = c.req.param('deckId');
+		const db = dbOf();
+
+		const [privateDeck] = await db
+			.select()
+			.from(decks)
+			.where(and(eq(decks.id, deckId), eq(decks.userId, userId)))
+			.limit(1);
+		if (!privateDeck) return c.json({ error: 'not_found' }, 404);
+		if (!privateDeck.forkedFromMarketplaceDeckId) {
+			return c.json({ error: 'not_a_fork' }, 422);
+		}
+		if (!privateDeck.forkedFromMarketplaceVersionId) {
+			return c.json({ error: 'fork_pointer_missing' }, 500);
+		}
+
+		const [sourceDeck] = await db
+			.select()
+			.from(publicDecks)
+			.where(eq(publicDecks.id, privateDeck.forkedFromMarketplaceDeckId))
+			.limit(1);
+		if (!sourceDeck) return c.json({ error: 'source_deck_gone' }, 404);
+		if (!sourceDeck.latestVersionId) return c.json({ error: 'no_published_version' }, 409);
+
+		// Already up-to-date?
+		if (sourceDeck.latestVersionId === privateDeck.forkedFromMarketplaceVersionId) {
+			return c.json({
+				up_to_date: true,
+				added: 0,
+				changed: 0,
+				removed: 0,
+			});
+		}
+
+		const [fromCards, toCards, latestVersion, fromVersion] = await Promise.all([
+			db
+				.select({ contentHash: publicDeckCards.contentHash, ord: publicDeckCards.ord })
+				.from(publicDeckCards)
+				.where(eq(publicDeckCards.versionId, privateDeck.forkedFromMarketplaceVersionId)),
+			loadVersionCards(db, sourceDeck.latestVersionId),
+			db
+				.select()
+				.from(publicDeckVersions)
+				.where(eq(publicDeckVersions.id, sourceDeck.latestVersionId))
+				.limit(1)
+				.then((rows) => rows[0] ?? null),
+			db
+				.select()
+				.from(publicDeckVersions)
+				.where(eq(publicDeckVersions.id, privateDeck.forkedFromMarketplaceVersionId))
+				.limit(1)
+				.then((rows) => rows[0] ?? null),
+		]);
+		if (!latestVersion || !fromVersion) {
+			return c.json({ error: 'version_resolution_failed' }, 500);
+		}
+
+		const diff = computeDiff({
+			from: fromCards,
+			to: toCards.map((card) => ({
+				contentHash: card.contentHash,
+				type: card.type,
+				fields: card.fields as Record<string, string>,
+				ord: card.ord,
+			})),
+			fromInfo: { semver: fromVersion.semver, versionId: fromVersion.id },
+			toInfo: { semver: latestVersion.semver, versionId: latestVersion.id },
+		});
+
+		// Apply: added + changed.next werden als neue private Cards
+		// eingefügt (mit neuem content_hash, da identisch zur
+		// Marketplace-Karte). Unveränderte Karten existieren schon
+		// privat — wir können den Insert sicher überspringen, weil
+		// (deck_id, content_hash) im privaten cards.cards einzigartig
+		// ist (gleiche Hash-Funktion via @cards/domain).
+		// Removed-Cards bleiben lokal (server-authoritative User-Choice).
+		const now = new Date();
+
+		// Existing private content-hashes lookup, damit wir keine
+		// Duplikate einfügen falls die Heuristik mal danebenliegt.
+		const existingPrivate = await db
+			.select({ contentHash: cards.contentHash })
+			.from(cards)
+			.where(and(eq(cards.deckId, deckId), eq(cards.userId, userId)));
+		const existingHashes = new Set(existingPrivate.map((row) => row.contentHash));
+
+		const toInsert = [...diff.added, ...diff.changed.map((entry) => entry.next)].filter(
+			(card) => !existingHashes.has(card.contentHash)
+		);
+
+		await db.transaction(async (tx) => {
+			for (const card of toInsert) {
+				const cardId = ulid();
+				// Content-Hash kommt aus Marketplace; wir haben die Karte
+				// schon nicht im privaten Set, also sicher zu inserten.
+				const computedHash = await cardContentHash({
+					type: card.type,
+					fields: card.fields,
+				});
+				await tx.insert(cards).values({
+					id: cardId,
+					deckId,
+					userId,
+					type: card.type,
+					fields: card.fields,
+					mediaRefs: [],
+					contentHash: computedHash,
+					createdAt: now,
+					updatedAt: now,
+				});
+
+				const subIndexes = subIndexCountFor(card.type, card.fields);
+				const initialReviews = buildInitialReviews(userId, cardId, subIndexes, now);
+				if (initialReviews.length > 0) {
+					await tx.insert(reviews).values(initialReviews);
+				}
+			}
+
+			// Update Fork-Pointer auf die neue Version.
+			await tx
+				.update(decks)
+				.set({
+					forkedFromMarketplaceVersionId: sourceDeck.latestVersionId,
+					updatedAt: now,
+				})
+				.where(eq(decks.id, deckId));
+		});
+
+		return c.json({
+			up_to_date: false,
+			from: { semver: fromVersion.semver, versionId: fromVersion.id },
+			to: { semver: latestVersion.semver, versionId: latestVersion.id },
+			added: diff.added.length,
+			changed: diff.changed.length,
+			removed: diff.removed.length,
+			cards_inserted: toInsert.length,
+		});
+	});
+
+	return r;
+}
diff --git a/apps/api/src/routes/marketplace/subscriptions.ts b/apps/api/src/routes/marketplace/subscriptions.ts
new file mode 100644
index 0000000..489695c
--- /dev/null
+++ b/apps/api/src/routes/marketplace/subscriptions.ts
@@ -0,0 +1,252 @@
+import { and, asc, eq } from 'drizzle-orm';
+import { Hono } from 'hono';
+
+import { getDb, type CardsDb } from '../../db/connection.ts';
+import {
+	deckSubscriptions,
+	publicDeckCards,
+	publicDeckVersions,
+	publicDecks,
+} from '../../db/schema/index.ts';
+import { authMiddleware, type AuthVars } from '../../middleware/auth.ts';
+import { optionalAuthMiddleware } from '../../middleware/marketplace/optional-auth.ts';
+import { computeDiff } from '../../lib/marketplace/diff.ts';
+
+/**
+ * Subscriptions + Version-Read + Smart-Merge-Diff.
+ *
+ * - `POST /decks/:slug/subscribe`         — Intent-Tracking. Wenn paid:
+ *                                            Purchase-Check (R3 dormant).
+ * - `DELETE /decks/:slug/subscribe`        — abbestellen.
+ * - `GET /me/subscriptions`               — eigene Subs + update-Indikator.
+ * - `GET /decks/:slug/versions/:semver`    — voller Version-Payload mit
+ *                                            Cards in stable ord-Order.
+ * - `GET /decks/:slug/diff?from=:semver`   — Smart-Merge-Payload.
+ *
+ * Geport aus
+ * `cards-decommission-base:services/cards-server/src/services/subscriptions.ts`.
+ */
+
+export type MarketplaceSubscriptionsDeps = { db?: CardsDb };
+
+async function findDeckBySlug(db: CardsDb, slug: string) {
+	const [row] = await db.select().from(publicDecks).where(eq(publicDecks.slug, slug)).limit(1);
+	return row ?? null;
+}
+
+async function findVersion(db: CardsDb, deckId: string, semver: string) {
+	const [row] = await db
+		.select()
+		.from(publicDeckVersions)
+		.where(and(eq(publicDeckVersions.deckId, deckId), eq(publicDeckVersions.semver, semver)))
+		.limit(1);
+	return row ?? null;
+}
+
+async function loadVersionCards(db: CardsDb, versionId: string) {
+	return db
+		.select()
+		.from(publicDeckCards)
+		.where(eq(publicDeckCards.versionId, versionId))
+		.orderBy(asc(publicDeckCards.ord));
+}
+
+export function subscriptionsRouter(
+	deps: MarketplaceSubscriptionsDeps = {}
+): Hono<{ Variables: Partial<AuthVars> }> {
+	const r = new Hono<{ Variables: Partial<AuthVars> }>();
+	const dbOf = () => deps.db ?? getDb();
+
+	// ─── Subscribe / Unsubscribe (auth — per-route middleware
+	// statt route-mount, damit die Public-Read-Routes weiter unten
+	// nicht vom Wildcard-Middleware des Auth-Subrouters gefangen werden) ──
+
+	r.post('/decks/:slug/subscribe', authMiddleware, async (c) => {
+		const userId = c.get('userId');
+		const slug = c.req.param('slug');
+		const db = dbOf();
+		const deck = await findDeckBySlug(db, slug);
+		if (!deck) return c.json({ error: 'not_found' }, 404);
+		if (deck.isTakedown) return c.json({ error: 'takedown_active' }, 403);
+		if (!deck.latestVersionId) {
+			return c.json({ error: 'no_published_version' }, 409);
+		}
+		// Paid-Decks brauchen einen Purchase-Beleg — Pipeline kommt mit
+		// Phase ζ wieder; bis dahin: nur Owner darf seinen eigenen paid
+		// Deck testweise subscriben.
+		if (deck.priceCredits > 0 && deck.ownerUserId !== userId) {
+			return c.json({ error: 'paid_deck_purchase_required' }, 402);
+		}
+
+		await db
+			.insert(deckSubscriptions)
+			.values({
+				userId,
+				deckId: deck.id,
+				currentVersionId: deck.latestVersionId,
+			})
+			.onConflictDoUpdate({
+				target: [deckSubscriptions.userId, deckSubscriptions.deckId],
+				set: { currentVersionId: deck.latestVersionId },
+			});
+
+		return c.json(
+			{
+				subscribed: true,
+				deck_slug: slug,
+				current_version_id: deck.latestVersionId,
+			},
+			201
+		);
+	});
+
+	r.delete('/decks/:slug/subscribe', authMiddleware, async (c) => {
+		const userId = c.get('userId');
+		const slug = c.req.param('slug');
+		const db = dbOf();
+		const deck = await findDeckBySlug(db, slug);
+		if (!deck) return c.json({ error: 'not_found' }, 404);
+		await db
+			.delete(deckSubscriptions)
+			.where(and(eq(deckSubscriptions.userId, userId), eq(deckSubscriptions.deckId, deck.id)));
+		return c.json({ subscribed: false });
+	});
+
+	r.get('/me/subscriptions', authMiddleware, async (c) => {
+		const userId = c.get('userId');
+		const rows = await dbOf()
+			.select({
+				deckSlug: publicDecks.slug,
+				deckTitle: publicDecks.title,
+				deckDescription: publicDecks.description,
+				deckLatestVersionId: publicDecks.latestVersionId,
+				subscribedAt: deckSubscriptions.subscribedAt,
+				notifyUpdates: deckSubscriptions.notifyUpdates,
+				currentVersionId: deckSubscriptions.currentVersionId,
+			})
+			.from(deckSubscriptions)
+			.innerJoin(publicDecks, eq(publicDecks.id, deckSubscriptions.deckId))
+			.where(eq(deckSubscriptions.userId, userId))
+			.orderBy(asc(deckSubscriptions.subscribedAt));
+
+		return c.json({
+			subscriptions: rows.map((row) => ({
+				deck_slug: row.deckSlug,
+				deck_title: row.deckTitle,
+				deck_description: row.deckDescription,
+				subscribed_at: row.subscribedAt.toISOString(),
+				notify_updates: row.notifyUpdates,
+				current_version_id: row.currentVersionId,
+				latest_version_id: row.deckLatestVersionId,
+				update_available:
+					row.deckLatestVersionId !== null && row.currentVersionId !== row.deckLatestVersionId,
+			})),
+		});
+	});
+
+	r.get('/decks/:slug/subscribe', authMiddleware, async (c) => {
+		const userId = c.get('userId');
+		const slug = c.req.param('slug');
+		const db = dbOf();
+		const rows = await db
+			.select({ id: deckSubscriptions.deckId, currentVersionId: deckSubscriptions.currentVersionId })
+			.from(deckSubscriptions)
+			.innerJoin(publicDecks, eq(publicDecks.id, deckSubscriptions.deckId))
+			.where(and(eq(deckSubscriptions.userId, userId), eq(publicDecks.slug, slug)))
+			.limit(1);
+		if (rows.length === 0) return c.json({ subscribed: false });
+		return c.json({ subscribed: true, current_version_id: rows[0].currentVersionId });
+	});
+
+	// ─── Public-Read: Version + Diff ─────────────────────────────────
+
+	r.get('/decks/:slug/versions/:semver', optionalAuthMiddleware, async (c) => {
+		const slug = c.req.param('slug');
+		const semver = c.req.param('semver');
+		const db = dbOf();
+		const deck = await findDeckBySlug(db, slug);
+		if (!deck) return c.json({ error: 'not_found' }, 404);
+		const version = await findVersion(db, deck.id, semver);
+		if (!version) return c.json({ error: 'version_not_found' }, 404);
+		const cards = await loadVersionCards(db, version.id);
+
+		return c.json({
+			version: {
+				id: version.id,
+				deck_id: version.deckId,
+				semver: version.semver,
+				changelog: version.changelog,
+				content_hash: version.contentHash,
+				card_count: version.cardCount,
+				published_at: version.publishedAt.toISOString(),
+			},
+			cards: cards.map((card) => ({
+				content_hash: card.contentHash,
+				type: card.type,
+				fields: card.fields,
+				ord: card.ord,
+			})),
+		});
+	});
+
+	r.get('/decks/:slug/diff', optionalAuthMiddleware, async (c) => {
+		const slug = c.req.param('slug');
+		const fromSemver = c.req.query('from');
+		if (!fromSemver) return c.json({ error: 'missing_query', detail: 'from=:semver' }, 422);
+
+		const db = dbOf();
+		const deck = await findDeckBySlug(db, slug);
+		if (!deck) return c.json({ error: 'not_found' }, 404);
+		if (!deck.latestVersionId) return c.json({ error: 'no_published_version' }, 409);
+
+		const [latest, from] = await Promise.all([
+			db
+				.select()
+				.from(publicDeckVersions)
+				.where(eq(publicDeckVersions.id, deck.latestVersionId))
+				.limit(1)
+				.then((rows) => rows[0] ?? null),
+			findVersion(db, deck.id, fromSemver),
+		]);
+		if (!latest) return c.json({ error: 'latest_version_missing' }, 500);
+		if (!from) return c.json({ error: 'from_version_not_found' }, 404);
+
+		// Empty diff bei Identität.
+		if (from.id === latest.id) {
+			return c.json({
+				from: { semver: fromSemver, versionId: from.id },
+				to: { semver: latest.semver, versionId: latest.id },
+				added: [],
+				changed: [],
+				unchanged: [],
+				removed: [],
+			});
+		}
+
+		const [fromCards, toCardsRaw] = await Promise.all([
+			db
+				.select({ contentHash: publicDeckCards.contentHash, ord: publicDeckCards.ord })
+				.from(publicDeckCards)
+				.where(eq(publicDeckCards.versionId, from.id)),
+			loadVersionCards(db, latest.id),
+		]);
+
+		const toCards = toCardsRaw.map((card) => ({
+			contentHash: card.contentHash,
+			type: card.type,
+			fields: card.fields as Record<string, string>,
+			ord: card.ord,
+		}));
+
+		const diff = computeDiff({
+			from: fromCards,
+			to: toCards,
+			fromInfo: { semver: fromSemver, versionId: from.id },
+			toInfo: { semver: latest.semver, versionId: latest.id },
+		});
+
+		return c.json(diff);
+	});
+
+	return r;
+}
diff --git a/apps/api/tests/marketplace-diff.test.ts b/apps/api/tests/marketplace-diff.test.ts
new file mode 100644
index 0000000..17cdf42
--- /dev/null
+++ b/apps/api/tests/marketplace-diff.test.ts
@@ -0,0 +1,104 @@
+import { describe, expect, it } from 'vitest';
+
+import { computeDiff } from '../src/lib/marketplace/diff.ts';
+
+const fromInfo = { semver: '1.0.0', versionId: 'v1' };
+const toInfo = { semver: '1.1.0', versionId: 'v2' };
+
+function fullCard(ord: number, hash: string, front = `Q${ord}`, back = `A${ord}`) {
+	return { contentHash: hash, type: 'basic', fields: { front, back }, ord };
+}
+
+describe('computeDiff', () => {
+	it('classifies all-unchanged when nothing moved', () => {
+		const diff = computeDiff({
+			from: [
+				{ contentHash: 'h1', ord: 0 },
+				{ contentHash: 'h2', ord: 1 },
+			],
+			to: [fullCard(0, 'h1'), fullCard(1, 'h2')],
+			fromInfo,
+			toInfo,
+		});
+		expect(diff.unchanged).toHaveLength(2);
+		expect(diff.added).toHaveLength(0);
+		expect(diff.changed).toHaveLength(0);
+		expect(diff.removed).toHaveLength(0);
+	});
+
+	it('detects added when a brand-new card appears', () => {
+		const diff = computeDiff({
+			from: [{ contentHash: 'h1', ord: 0 }],
+			to: [fullCard(0, 'h1'), fullCard(1, 'h2')],
+			fromInfo,
+			toInfo,
+		});
+		expect(diff.added).toHaveLength(1);
+		expect(diff.added[0].contentHash).toBe('h2');
+		expect(diff.unchanged).toHaveLength(1);
+	});
+
+	it('detects removed when a card vanishes (and ord is unique)', () => {
+		const diff = computeDiff({
+			from: [
+				{ contentHash: 'h1', ord: 0 },
+				{ contentHash: 'h2', ord: 1 },
+			],
+			to: [fullCard(0, 'h1')],
+			fromInfo,
+			toInfo,
+		});
+		expect(diff.removed).toHaveLength(1);
+		expect(diff.removed[0].contentHash).toBe('h2');
+	});
+
+	it('detects changed when same ord has different hash', () => {
+		const diff = computeDiff({
+			from: [{ contentHash: 'h1', ord: 0 }],
+			to: [fullCard(0, 'h1-tweaked', 'Q0', 'A0-edited')],
+			fromInfo,
+			toInfo,
+		});
+		expect(diff.changed).toHaveLength(1);
+		expect(diff.changed[0].previous.contentHash).toBe('h1');
+		expect(diff.changed[0].next.contentHash).toBe('h1-tweaked');
+		expect(diff.added).toHaveLength(0);
+		expect(diff.removed).toHaveLength(0);
+	});
+
+	it('mixed: 1 unchanged, 1 changed, 1 added, 1 removed', () => {
+		const diff = computeDiff({
+			from: [
+				{ contentHash: 'h-stay', ord: 0 },
+				{ contentHash: 'h-old', ord: 1 },
+				{ contentHash: 'h-bye', ord: 2 },
+			],
+			to: [
+				fullCard(0, 'h-stay'),
+				fullCard(1, 'h-new', 'replaced', 'card'),
+				fullCard(3, 'h-fresh', 'new', 'card'),
+			],
+			fromInfo,
+			toInfo,
+		});
+		expect(diff.unchanged.map((c) => c.contentHash)).toEqual(['h-stay']);
+		expect(diff.changed).toHaveLength(1);
+		expect(diff.changed[0].previous.contentHash).toBe('h-old');
+		expect(diff.changed[0].next.contentHash).toBe('h-new');
+		expect(diff.added).toHaveLength(1);
+		expect(diff.added[0].contentHash).toBe('h-fresh');
+		expect(diff.removed).toHaveLength(1);
+		expect(diff.removed[0].contentHash).toBe('h-bye');
+	});
+
+	it('returns the version-info verbatim', () => {
+		const diff = computeDiff({
+			from: [],
+			to: [],
+			fromInfo,
+			toInfo,
+		});
+		expect(diff.from).toEqual(fromInfo);
+		expect(diff.to).toEqual(toInfo);
+	});
+});