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/lib/server/pocketbase.ts

@@ -1219,6 +1219,47 @@ export async function getBooksWithAudioCount(limit = 100): Promise<AudioBookEntr
 	return entries.slice(0, limit);
 }
 
+/**
+ * Returns a map of chapter number → best available voice for a given slug.
+ * "Best" means: prefer `preferredVoice` if a done job exists for it,
+ * otherwise fall back to any done voice for that chapter.
+ * Result is cached per slug for 60 seconds (audio jobs complete frequently).
+ */
+export async function getReadyChaptersForSlug(
+	slug: string,
+	preferredVoice = ''
+): Promise<Map<number, string>> {
+	const cacheKey = `audio:ready_chapters:${slug}`;
+	const cached = await cache.get<{ chapter: number; voice: string }[]>(cacheKey);
+	const raw = cached ?? await (async () => {
+		const filter = encodeURIComponent(`slug="${slug.replace(/"/g, '\\"')}"&&status="done"`);
+		const jobs = await listAll<AudioJob>('audio_jobs', filter, 'chapter');
+		const result: { chapter: number; voice: string }[] = jobs.map((j) => ({
+			chapter: j.chapter,
+			voice: j.voice ?? ''
+		}));
+		await cache.set(cacheKey, result, 60);
+		return result;
+	})();
+
+	// Build chapter → voices map
+	const byChapter = new Map<number, string[]>();
+	for (const { chapter, voice } of raw) {
+		if (!byChapter.has(chapter)) byChapter.set(chapter, []);
+		byChapter.get(chapter)!.push(voice);
+	}
+
+	// Resolve best voice per chapter
+	const result = new Map<number, string>();
+	for (const [chapter, voices] of byChapter) {
+		const best = preferredVoice && voices.includes(preferredVoice)
+			? preferredVoice
+			: voices[0] ?? '';
+		result.set(chapter, best);
+	}
+	return result;
+}
+
 // ─── Translation jobs ─────────────────────────────────────────────────────────
 
 export interface TranslationJob {