feat(discover): Tinder-style book discovery + fix duplicate books

- New /discover page with swipe UI: left=skip, right=like, up=read now, down=nope
- Onboarding modal to collect genre/status preferences (persisted in localStorage)
- 3-card stack with pointer-event drag, CSS fly-out animation, 5 action buttons
- Tap card for preview modal; empty state with deck reset
- Like/read-now auto-saves book to user library
- POST /api/discover/vote + DELETE for deck reset
- Discovery vote persistence via PocketBase discovery_votes collection
- Fix duplicate books: dedup by slug in getBooksBySlugs
- Fix WriteMetadata TOCTOU race: conflict-retry on concurrent insert

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

ui/src/routes/discover/+page.server.ts

@@ -0,0 +1,14 @@
+import type { PageServerLoad } from './$types';
+import { getBooksForDiscovery } from '$lib/server/pocketbase';
+import type { DiscoveryPrefs } from '$lib/server/pocketbase';
+
+export const load: PageServerLoad = async ({ locals, url }) => {
+	let prefs: DiscoveryPrefs | undefined;
+	const prefsParam = url.searchParams.get('prefs');
+	if (prefsParam) {
+		try { prefs = JSON.parse(prefsParam) as DiscoveryPrefs; } catch { /* ignore */ }
+	}
+
+	const books = await getBooksForDiscovery(locals.sessionId, locals.user?.id, prefs).catch(() => []);
+	return { books };
+};