surface audio-ready chapters: headphones badge on chapter list, instant-play prompt on reader

- getReadyChaptersForSlug(slug, preferredVoice) in pocketbase.ts: per-slug done jobs map, cached 60s, prefers user's voice
- GET /api/audio/chapters?slug=&voice= endpoint
- Chapter list (/books/[slug]/chapters): amber headphones icon on ready rows, play button for instant listen, banner showing ready count
- Chapter reader: audioReady + availableVoice from server load; 'Audio ready — Listen now' banner shown when audio exists and player is not yet active; sets voice preference before expanding player

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

@@ -0,0 +1,18 @@
+import { json, error } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import { getReadyChaptersForSlug } from '$lib/server/pocketbase';
+
+export const GET: RequestHandler = async ({ url }) => {
+	const slug = url.searchParams.get('slug') ?? '';
+	if (!slug) error(400, 'slug is required');
+
+	const voice = url.searchParams.get('voice') ?? '';
+	const readyMap = await getReadyChaptersForSlug(slug, voice);
+
+	// Return array of { chapter, voice } pairs
+	const chapters = [...readyMap.entries()].map(([chapter, v]) => ({ chapter, voice: v }));
+
+	return json({ chapters }, {
+		headers: { 'Cache-Control': 'public, max-age=60' }
+	});
+};