perf(ui): eliminate full listBooks() scan on every page load

The home, library, and /books routes were fetching all 15k books from
PocketBase on every SSR request (31 sequential HTTP calls per request).

Changes:
- Add src/lib/server/cache.ts: generic Valkey JSON cache
- Add getBooksBySlugs(): single PB query fetching only requested slugs,
  with fallback to the 5-min Valkey cache populated by listBooks()
- listBooks(): now caches results in Valkey for 5 min (safety net for
  admin routes that still need the full list)
- Home + /api/home: replaced listBooks()+filter with getBooksBySlugs()
  on progress slugs only — typically 1 PB request instead of 31
- /books + /api/library: same pattern using progress+saved slug union

ui/src/lib/server/pocketbase.ts

@@ -6,6 +6,7 @@
 
 import { env } from '$env/dynamic/private';
 import { log } from '$lib/server/logger';
+import * as cache from '$lib/server/cache';
 
 const PB_URL = env.POCKETBASE_URL ?? 'http://localhost:8090';
 const PB_EMAIL = env.POCKETBASE_ADMIN_EMAIL ?? 'admin@libnovel.local';
@@ -200,16 +201,65 @@ async function listOne<T>(collection: string, filter: string): Promise<T | null>
 
 // ─── Books ────────────────────────────────────────────────────────────────────
 
+const BOOKS_CACHE_KEY = 'books:all';
+const BOOKS_CACHE_TTL = 5 * 60; // 5 minutes
+
 export async function listBooks(): Promise<Book[]> {
+	const cached = await cache.get<Book[]>(BOOKS_CACHE_KEY);
+	if (cached) {
+		log.debug('pocketbase', 'listBooks cache hit', { total: cached.length });
+		return cached;
+	}
 	const books = await listAll<Book>('books', '', '+title');
 	const nullTitles = books.filter((b) => b.title == null).length;
 	if (nullTitles > 0) {
 		log.warn('pocketbase', 'listBooks: books with null title', { count: nullTitles, total: books.length });
 	}
-	log.debug('pocketbase', 'listBooks', { total: books.length, nullTitles });
+	log.debug('pocketbase', 'listBooks cache miss', { total: books.length, nullTitles });
+	await cache.set(BOOKS_CACHE_KEY, books, BOOKS_CACHE_TTL);
 	return books;
 }
 
+/**
+ * Fetch only the books whose slugs are in the given set.
+ * Uses PocketBase filter `slug IN (...)` — a single request regardless of how
+ * many slugs are requested. Falls back to empty array on error.
+ *
+ * Use this instead of listBooks() whenever you only need a small subset of
+ * books (e.g. the user's reading list or saved shelf).
+ *
+ * PocketBase filter syntax for IN: slug='a' || slug='b' || ...
+ * Limited to 200 slugs to keep the filter URL sane; callers with larger sets
+ * should fall back to listBooks().
+ */
+export async function getBooksBySlugs(slugs: Iterable<string>): Promise<Book[]> {
+	const slugArr = [...new Set(slugs)].slice(0, 200);
+	if (slugArr.length === 0) return [];
+
+	// Check cache for each slug individually (populated by prior listBooks calls).
+	// If all slugs hit, skip the network round-trip entirely.
+	const cached = await cache.get<Book[]>(BOOKS_CACHE_KEY);
+	if (cached) {
+		const slugSet = new Set(slugArr);
+		const found = cached.filter((b) => slugSet.has(b.slug));
+		if (found.length === slugArr.length) {
+			log.debug('pocketbase', 'getBooksBySlugs cache hit', { count: found.length });
+			return found;
+		}
+	}
+
+	// Build filter: slug='a' || slug='b' || ...
+	const filter = slugArr.map((s) => `slug='${s.replace(/'/g, "\\'")}'`).join(' || ');
+	const books = await listAll<Book>('books', filter, '+title');
+	log.debug('pocketbase', 'getBooksBySlugs', { requested: slugArr.length, found: books.length });
+	return books;
+}
+
+/** Invalidate the books cache (call after a book is created/updated/deleted). */
+export async function invalidateBooksCache(): Promise<void> {
+	await cache.invalidate(BOOKS_CACHE_KEY);
+}
+
 export async function getBook(slug: string): Promise<Book | null> {
 	return listOne<Book>('books', `slug="${slug}"`);
 }