feat(cards): Phase ε.4 — Card list + discussions on /d/<slug>

- DiscussionService.countsForDeck: bulk count (visible) comments per
  card-content-hash for one deck. Mounted at GET
  /v1/decks/:slug/discussion-counts so the public deck page can
  render comment badges without N+1 fetches.
- <DeckCardList> on /d/<slug>: lists the latest version's cards,
  renders a one-line preview + "💬 N" badge, and expands the
  inline <CardDiscussions> on click. Anonymous visitors see counts;
  posting requires auth (CardDiscussions already gates that).

services/cards-server/src/routes/discussions.ts

@@ -23,6 +23,11 @@ export function createDiscussionRoutes(service: DiscussionService) {
 		return c.json(list);
 	});
 
+	router.get('/decks/:slug/discussion-counts', async (c) => {
+		const counts = await service.countsForDeck(c.req.param('slug'));
+		return c.json(counts);
+	});
+
 	router.post('/cards/:contentHash/discussions', async (c) => {
 		const user = requireUser(c.get('user'));
 		const parsed = postSchema.safeParse(await c.req.json().catch(() => ({})));

services/cards-server/src/services/discussions.ts

@@ -9,7 +9,7 @@
  * if we want full nesting later it's already there.
  */
 
-import { and, asc, eq } from 'drizzle-orm';
+import { and, asc, eq, sql } from 'drizzle-orm';
 import type { Database } from '../db/connection';
 import { cardDiscussions, publicDecks } from '../db/schema';
 import { ForbiddenError, NotFoundError } from '../lib/errors';
@@ -52,6 +52,31 @@ export class DiscussionService {
 		return row;
 	}
 
+	/**
+	 * Bulk count of (visible) comments per card-content-hash for one
+	 * deck — powers the "Karten" overview on the public deck page so
+	 * we don't fan out one request per card.
+	 */
+	async countsForDeck(deckSlug: string): Promise<Record<string, number>> {
+		const deck = await this.db.query.publicDecks.findFirst({
+			where: eq(publicDecks.slug, deckSlug),
+		});
+		if (!deck) throw new NotFoundError('Deck not found');
+
+		const rows = await this.db
+			.select({
+				contentHash: cardDiscussions.cardContentHash,
+				count: sql<number>`count(*)::int`.as('count'),
+			})
+			.from(cardDiscussions)
+			.where(and(eq(cardDiscussions.deckId, deck.id), eq(cardDiscussions.hidden, false)))
+			.groupBy(cardDiscussions.cardContentHash);
+
+		const out: Record<string, number> = {};
+		for (const r of rows) out[r.contentHash] = r.count;
+		return out;
+	}
+
 	async listForCard(cardContentHash: string) {
 		const rows = await this.db
 			.select()