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/routes/+page.server.ts

@@ -1,6 +1,6 @@
 import type { PageServerLoad } from './$types';
 import {
-	listBooks,
+	getBooksBySlugs,
 	recentlyAddedBooks,
 	allProgress,
 	getHomeStats,
@@ -10,14 +10,15 @@ import { log } from '$lib/server/logger';
 import type { Book, Progress } from '$lib/server/pocketbase';
 
 export const load: PageServerLoad = async ({ locals }) => {
-	let allBooks: Book[] = [];
+	// Step 1: fetch progress + recent books + stats in parallel.
+	// We intentionally do NOT call listBooks() here — we only need books that
+	// appear in the user's progress list, which is a tiny subset of 15k books.
 	let recentBooks: Book[] = [];
 	let progressList: Progress[] = [];
 	let stats = { totalBooks: 0, totalChapters: 0 };
 
 	try {
-		[allBooks, recentBooks, progressList, stats] = await Promise.all([
-			listBooks(),
+		[recentBooks, progressList, stats] = await Promise.all([
 			recentlyAddedBooks(8),
 			allProgress(locals.sessionId, locals.user?.id),
 			getHomeStats()
@@ -26,8 +27,14 @@ export const load: PageServerLoad = async ({ locals }) => {
 		log.error('home', 'failed to load home data', { err: String(e) });
 	}
 
-	// Build slug → book lookup
-	const bookMap = new Map<string, Book>(allBooks.map((b) => [b.slug, b]));
+	// Step 2: fetch only the books we actually need for continue-reading.
+	// This is O(progress entries) instead of O(15k books).
+	const progressSlugs = progressList.map((p) => p.slug);
+	const progressBooks = progressSlugs.length > 0
+		? await getBooksBySlugs(progressSlugs).catch(() => [] as Book[])
+		: [];
+
+	const bookMap = new Map<string, Book>(progressBooks.map((b) => [b.slug, b]));
 
 	// Continue reading: progress entries joined with book data, most recent first
 	const continueReading = progressList