refactor(api): DTO-Helper extrahieren + N+1 in marketplace/decks beheben

- `lib/dto.ts`: `toDeckDto` und `toCardDto` aus routes/decks.ts und
  routes/cards.ts extrahiert — testbar, zentrale Output-Shape-Doku
- `lib/marketplace/dto.ts`: `toPublicDeckDto`, `toOwnerDto`, `toVersionDto`
  aus routes/marketplace/decks.ts extrahiert
- `GET /:slug` in marketplace/decks.ts: Version + Owner parallel per
  `Promise.all` statt sequenziell (2 RTT → 1 RTT)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/api/src/lib/dto.ts

@@ -0,0 +1,33 @@
+import { cards, decks } from '../db/schema/index.ts';
+
+export function toDeckDto(row: typeof decks.$inferSelect) {
+	return {
+		id: row.id,
+		user_id: row.userId,
+		name: row.name,
+		description: row.description,
+		color: row.color,
+		category: row.category,
+		visibility: row.visibility,
+		fsrs_settings: row.fsrsSettings,
+		content_hash: row.contentHash,
+		forked_from_marketplace_deck_id: row.forkedFromMarketplaceDeckId,
+		forked_from_marketplace_version_id: row.forkedFromMarketplaceVersionId,
+		created_at: row.createdAt.toISOString(),
+		updated_at: row.updatedAt.toISOString(),
+	};
+}
+
+export function toCardDto(row: typeof cards.$inferSelect) {
+	return {
+		id: row.id,
+		deck_id: row.deckId,
+		user_id: row.userId,
+		type: row.type,
+		fields: row.fields,
+		media_refs: row.mediaRefs ?? [],
+		content_hash: row.contentHash,
+		created_at: row.createdAt.toISOString(),
+		updated_at: row.updatedAt.toISOString(),
+	};
+}

apps/api/src/lib/marketplace/dto.ts

@@ -0,0 +1,43 @@
+import { publicDecks, publicDeckVersions } from '../../db/schema/index.ts';
+import type { AuthorRow } from '../../db/schema/marketplace/index.ts';
+
+export function toPublicDeckDto(row: typeof publicDecks.$inferSelect) {
+	return {
+		id: row.id,
+		slug: row.slug,
+		title: row.title,
+		description: row.description,
+		language: row.language,
+		category: row.category,
+		license: row.license,
+		price_credits: row.priceCredits,
+		owner_user_id: row.ownerUserId,
+		latest_version_id: row.latestVersionId,
+		is_featured: row.isFeatured,
+		is_takedown: row.isTakedown,
+		created_at: row.createdAt.toISOString(),
+	};
+}
+
+export function toOwnerDto(row: AuthorRow) {
+	return {
+		slug: row.slug,
+		display_name: row.displayName,
+		verified_mana: row.verifiedMana,
+		verified_community: row.verifiedCommunity,
+		pseudonym: row.pseudonym,
+	};
+}
+
+export function toVersionDto(row: typeof publicDeckVersions.$inferSelect) {
+	return {
+		id: row.id,
+		deck_id: row.deckId,
+		semver: row.semver,
+		changelog: row.changelog,
+		content_hash: row.contentHash,
+		card_count: row.cardCount,
+		published_at: row.publishedAt.toISOString(),
+		deprecated_at: row.deprecatedAt?.toISOString() ?? null,
+	};
+}