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/api/home/+server.ts

@@ -1,7 +1,7 @@
 import { json } from '@sveltejs/kit';
 import type { RequestHandler } from './$types';
 import {
-	listBooks,
+	getBooksBySlugs,
 	recentlyAddedBooks,
 	allProgress,
 	getHomeStats,
@@ -17,14 +17,12 @@ import type { Book, Progress } from '$lib/server/pocketbase';
  * Requires authentication (enforced by layout guard).
  */
 export const GET: RequestHandler = async ({ locals }) => {
-	let allBooks: Book[] = [];
 	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()
@@ -33,7 +31,13 @@ export const GET: RequestHandler = async ({ locals }) => {
 		log.error('api/home', 'failed to load home data', { err: String(e) });
 	}
 
-	const bookMap = new Map<string, Book>(allBooks.map((b) => [b.slug, b]));
+	// Fetch only the books we actually need for continue-reading.
+	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]));
 
 	const continueReading = progressList
 		.filter((p) => bookMap.has(p.slug))

ui/src/routes/api/library/+server.ts

@@ -1,7 +1,8 @@
 import { json } from '@sveltejs/kit';
 import type { RequestHandler } from './$types';
-import { listBooks, allProgress, getSavedSlugs } from '$lib/server/pocketbase';
+import { getBooksBySlugs, allProgress, getSavedSlugs } from '$lib/server/pocketbase';
 import { log } from '$lib/server/logger';
+import type { Book } from '$lib/server/pocketbase';
 
 /**
  * GET /api/library
@@ -11,23 +12,25 @@ import { log } from '$lib/server/logger';
  * Response shape mirrors LibraryItem in the iOS APIClient.
  */
 export const GET: RequestHandler = async ({ locals }) => {
-	let allBooks: Awaited<ReturnType<typeof listBooks>>;
-	let progressList: Awaited<ReturnType<typeof allProgress>>;
-	let savedSlugs: Set<string>;
+	let progressList: Awaited<ReturnType<typeof allProgress>> = [];
+	let savedSlugs: Set<string> = new Set();
 
 	try {
-		[allBooks, progressList, savedSlugs] = await Promise.all([
-			listBooks(),
+		[progressList, savedSlugs] = await Promise.all([
 			allProgress(locals.sessionId, locals.user?.id),
 			getSavedSlugs(locals.sessionId, locals.user?.id)
 		]);
 	} catch (e) {
 		log.error('api/library', 'failed to load library data', { err: String(e) });
-		allBooks = [];
-		progressList = [];
-		savedSlugs = new Set();
 	}
 
+	// Fetch only the books the user actually has in their library.
+	const progressSlugs = new Set(progressList.map((p) => p.slug));
+	const allNeededSlugs = new Set([...progressSlugs, ...savedSlugs]);
+	const books = allNeededSlugs.size > 0
+		? await getBooksBySlugs(allNeededSlugs).catch(() => [] as Book[])
+		: [];
+
 	const progressMap: Record<string, number> = {};
 	const progressUpdatedMap: Record<string, string> = {};
 	for (const p of progressList) {
@@ -35,9 +38,6 @@ export const GET: RequestHandler = async ({ locals }) => {
 		progressUpdatedMap[p.slug] = p.updated;
 	}
 
-	const progressSlugs = new Set(progressList.map((p) => p.slug));
-	const books = allBooks.filter((b) => progressSlugs.has(b.slug) || savedSlugs.has(b.slug));
-
 	const withProgress = books.filter((b) => progressSlugs.has(b.slug));
 	const savedOnly = books
 		.filter((b) => !progressSlugs.has(b.slug))