perf: fix discover page 4s load — parallel fetches + per-user caching

Three compounding issues caused the 4+ second load:

1. getAllRatings() ran sequentially after the first Promise.all group,
   adding it unnecessarily to the critical path. Now runs in parallel
   with listBooks/getVotedSlugs/getSavedSlugs (all 4 concurrent).

2. discovery_votes was fetched twice on every page load — once inside
   getBooksForDiscovery (via getVotedSlugs) and again by getVotedBooks.
   Fixed by caching getVotedSlugs results with a 30s TTL so the second
   call hits cache instead of PocketBase.

3. getVotedSlugs and getSavedSlugs were always uncached, hitting
   PocketBase on every navigation. Added short-TTL per-user Valkey
   cache entries (voted: 30s, saved: 60s). Cache is invalidated
   immediately after each write (upsertDiscoveryVote, clearDiscoveryVotes,
   undoDiscoveryVote, saveBook) so stale data is never served.

ui/src/lib/server/pocketbase.ts

@@ -659,11 +659,16 @@ function libraryFilter(sessionId: string, userId?: string): string {
 
 /** Returns all slugs the user has explicitly saved to their library. */
 export async function getSavedSlugs(sessionId: string, userId?: string): Promise<Set<string>> {
+	const cacheKey = userId ? `saved_slugs:user:${userId}` : `saved_slugs:session:${sessionId}`;
+	const cached = await cache.get<string[]>(cacheKey);
+	if (cached) return new Set(cached);
 	const rows = await listAll<UserLibraryEntry>(
 		'user_library',
 		libraryFilter(sessionId, userId)
 	);
-	return new Set(rows.map((r) => r.slug));
+	const slugs = rows.map((r) => r.slug);
+	await cache.set(cacheKey, slugs, SAVED_SLUGS_TTL);
+	return new Set(slugs);
 }
 
 /** Returns whether a specific slug is saved. */
@@ -710,7 +715,11 @@ export async function saveBook(
 	if (!res.ok) {
 		const body = await res.text().catch(() => '');
 		log.error('pocketbase', 'saveBook POST failed', { slug, status: res.status, body });
+		return;
 	}
+	// Invalidate saved-slugs cache so the next discover load excludes this book.
+	const savedKey = userId ? `saved_slugs:user:${userId}` : `saved_slugs:session:${sessionId}`;
+	await cache.invalidate(savedKey);
 }
 
 /** Remove a book from the user's library. */
@@ -2151,12 +2160,27 @@ function discoveryFilter(sessionId: string, userId?: string): string {
 	return `session_id="${sessionId}"`;
 }
 
+/** Cache TTL (seconds) for per-user voted/saved slug sets. Short — changes on every swipe. */
+const VOTED_SLUGS_TTL = 30;
+const SAVED_SLUGS_TTL = 60;
+
 export async function getVotedSlugs(sessionId: string, userId?: string): Promise<Set<string>> {
+	const cacheKey = userId ? `discovery_votes:user:${userId}` : `discovery_votes:session:${sessionId}`;
+	const cached = await cache.get<string[]>(cacheKey);
+	if (cached) return new Set(cached);
 	const rows = await listAll<DiscoveryVote>(
 		'discovery_votes',
 		discoveryFilter(sessionId, userId)
 	).catch(() => [] as DiscoveryVote[]);
-	return new Set(rows.map((r) => r.slug));
+	const slugs = rows.map((r) => r.slug);
+	await cache.set(cacheKey, slugs, VOTED_SLUGS_TTL);
+	return new Set(slugs);
+}
+
+/** Invalidate the voted-slugs cache entry after a vote is recorded. */
+async function invalidateVotedSlugsCache(sessionId: string, userId?: string): Promise<void> {
+	const key = userId ? `discovery_votes:user:${userId}` : `discovery_votes:session:${sessionId}`;
+	await cache.invalidate(key);
 }
 
 export async function upsertDiscoveryVote(
@@ -2179,6 +2203,7 @@ export async function upsertDiscoveryVote(
 		const res = await pbPost('/api/collections/discovery_votes/records', payload);
 		if (!res.ok) log.warn('pocketbase', 'upsertDiscoveryVote POST failed', { slug, status: res.status });
 	}
+	await invalidateVotedSlugsCache(sessionId, userId);
 }
 
 export async function clearDiscoveryVotes(sessionId: string, userId?: string): Promise<void> {
@@ -2189,6 +2214,7 @@ export async function clearDiscoveryVotes(sessionId: string, userId?: string): P
 			pbDelete(`/api/collections/discovery_votes/records/${r.id}`).catch(() => {})
 		)
 	);
+	await invalidateVotedSlugsCache(sessionId, userId);
 }
 
 // ─── Ratings ──────────────────────────────────────────────────────────────────
@@ -2283,10 +2309,13 @@ export async function getBooksForDiscovery(
 	userId?: string,
 	prefs?: DiscoveryPrefs
 ): Promise<Book[]> {
-	const [allBooks, votedSlugs, savedSlugs] = await Promise.all([
+	// Fetch all 4 independent data sources in parallel — previously getAllRatings
+	// ran sequentially after the first group, adding it to the critical path.
+	const [allBooks, votedSlugs, savedSlugs, ratingRows] = await Promise.all([
 		listBooks(),
 		getVotedSlugs(sessionId, userId),
-		getSavedSlugs(sessionId, userId)
+		getSavedSlugs(sessionId, userId),
+		getAllRatings(),
 	]);
 
 	let candidates = allBooks.filter((b) => !votedSlugs.has(b.slug) && !savedSlugs.has(b.slug));
@@ -2305,10 +2334,7 @@ export async function getBooksForDiscovery(
 		if (sf.length >= 3) candidates = sf;
 	}
 
-	// Fetch avg ratings for candidates, weight top-rated books to surface earlier.
-	// Fetch in one shot for all candidate slugs. Low-rated / unrated books still
-	// appear — they're just pushed further back via a stable sort before shuffle.
-	const ratingRows = await getAllRatings();
+	// Build slug→avg rating map
 	const ratingMap = new Map<string, { sum: number; count: number }>();
 	for (const r of ratingRows) {
 		const cur = ratingMap.get(r.slug) ?? { sum: 0, count: 0 };
@@ -2384,6 +2410,7 @@ export async function undoDiscoveryVote(
 	if (row) {
 		await pbDelete(`/api/collections/discovery_votes/records/${row.id}`).catch(() => {});
 	}
+	await invalidateVotedSlugsCache(sessionId, userId);
 }
 
 // ─── User stats ────────────────────────────────────────────────────────────────