add Ready to Listen feature: audio book shelf on home + /listen browse page

- getBooksWithAudioCount() in pocketbase.ts aggregates done audio_jobs, deduplicates by chapter per slug, caches 5 min
- GET /api/audio/books endpoint
- home page: readyToListen shelf with headphones badge, chapter count, Listen button, hideable
- /listen page: full grid with search, sort (most narrated / A-Z / recent), empty state

ui/src/routes/api/audio/books/+server.ts

@@ -0,0 +1,17 @@
+import { json } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import { getBooksWithAudioCount } from '$lib/server/pocketbase';
+
+/**
+ * GET /api/audio/books
+ * Returns books that have at least one completed narrated chapter,
+ * sorted by number of narrated chapters descending.
+ * Cached 5 minutes at the CDN/proxy level.
+ */
+export const GET: RequestHandler = async () => {
+	const entries = await getBooksWithAudioCount(100).catch(() => []);
+	return json(
+		{ books: entries },
+		{ headers: { 'Cache-Control': 'public, max-age=300' } }
+	);
+};