Add async audio generation: job tracking in PocketBase + UI polling

Replace blocking POST /api/audio with a non-blocking 202 flow: the Go
handler immediately enqueues a job in a new `audio_jobs` PocketBase
collection and returns {job_id, status}. A background goroutine runs
the actual Kokoro TTS work and updates job status (pending → generating
→ done/failed). A new GET /api/audio/status/{slug}/{n} endpoint lets
clients poll progress. The SvelteKit proxy and AudioPlayer.svelte are
updated to POST, then poll the status route every 2s until done.

ui/src/lib/components/AudioPlayer.svelte

@@ -331,6 +331,51 @@
 		return data.url;
 	}
 
+	type AudioStatusResponse =
+		| { status: 'done'; url: string; filename: string }
+		| { status: 'pending' | 'generating'; job_id: string }
+		| { status: 'idle' }
+		| { status: 'failed'; error?: string };
+
+	/**
+	 * Poll GET /api/audio/status/[slug]/[n]?voice=... every `intervalMs` ms
+	 * until status is "done" or "failed" (or the caller cancels via signal).
+	 *
+	 * Returns the final status response, or throws on network error / cancellation.
+	 */
+	async function pollAudioStatus(
+		targetSlug: string,
+		targetChapter: number,
+		targetVoice: string,
+		intervalMs = 2000,
+		signal?: AbortSignal
+	): Promise<AudioStatusResponse> {
+		const qs = new URLSearchParams();
+		if (targetVoice) qs.set('voice', targetVoice);
+		const url = `/api/audio/status/${targetSlug}/${targetChapter}?${qs.toString()}`;
+
+		while (true) {
+			if (signal?.aborted) throw new DOMException('Aborted', 'AbortError');
+
+			const res = await fetch(url, { signal });
+			if (!res.ok) throw new Error(`Status poll HTTP ${res.status}`);
+			const data = (await res.json()) as AudioStatusResponse;
+
+			if (data.status === 'done' || data.status === 'failed') {
+				return data;
+			}
+
+			// Still pending/generating — wait then retry.
+			await new Promise<void>((resolve, reject) => {
+				const timer = setTimeout(resolve, intervalMs);
+				signal?.addEventListener('abort', () => {
+					clearTimeout(timer);
+					reject(new DOMException('Aborted', 'AbortError'));
+				});
+			});
+		}
+	}
+
 	// ── Pre-fetch next chapter ─────────────────────────────────────────────────
 
 	async function prefetchNext() {
@@ -354,7 +399,7 @@
 				return;
 			}
 
-			// Slow path: trigger Kokoro generation in background
+			// Slow path: trigger Kokoro generation (non-blocking POST), then poll.
 			const res = await fetch(`/api/audio/${slug}/${nextChapter}`, {
 				method: 'POST',
 				headers: { 'Content-Type': 'application/json' },
@@ -362,13 +407,31 @@
 			});
 			if (!res.ok) throw new Error(`Prefetch generation failed: HTTP ${res.status}`);
 
+			// If the scraper returned cached audio immediately (200), use the url.
+			if (res.status === 200) {
+				const cached = (await res.json()) as { url: string };
+				stopNextProgress();
+				audioStore.nextProgress = 100;
+				audioStore.nextAudioUrl = cached.url;
+				audioStore.nextStatus = 'prefetched';
+				return;
+			}
+
+			// 202: poll until done.
+			const final = await pollAudioStatus(slug, nextChapter, voice);
 			stopNextProgress();
 			audioStore.nextProgress = 100;
 
-			const url2 = await tryPresign(slug, nextChapter, voice);
-			if (!url2) throw new Error('Prefetch: audio generated but presign returned 404');
+			if (final.status === 'failed') {
+				throw new Error(`Prefetch failed: ${(final as { error?: string }).error ?? 'unknown'}`);
+			}
 
-			audioStore.nextAudioUrl = url2;
+			// Use the URL from the status response, or fall back to presign.
+			const doneUrl =
+				(final as { url?: string }).url ?? (await tryPresign(slug, nextChapter, voice));
+			if (!doneUrl) throw new Error('Prefetch: audio done but no URL available');
+
+			audioStore.nextAudioUrl = doneUrl;
 			audioStore.nextStatus = 'prefetched';
 		} catch {
 			stopNextProgress();
@@ -447,7 +510,7 @@
 				return;
 			}
 
-			// Slow path: trigger Kokoro generation.
+			// Slow path: trigger Kokoro generation (non-blocking POST), then poll.
 			audioStore.status = 'generating';
 			startProgress();
 
@@ -458,11 +521,32 @@
 			});
 			if (!res.ok) throw new Error(`Generation failed: HTTP ${res.status}`);
 
+			// If the scraper returned cached audio immediately (200), use the url.
+			if (res.status === 200) {
+				const cached = (await res.json()) as { url: string };
+				await finishProgress();
+				audioStore.audioUrl = cached.url;
+				audioStore.status = 'ready';
+				maybeStartPrefetch();
+				return;
+			}
+
+			// 202: poll until done.
+			const final = await pollAudioStatus(slug, chapter, voice);
+
+			if (final.status === 'failed') {
+				throw new Error(
+					`Generation failed: ${(final as { error?: string }).error ?? 'unknown error'}`
+				);
+			}
+
 			await finishProgress();
 
-			const url2 = await tryPresign(slug, chapter, voice);
-			if (!url2) throw new Error('Audio generated but presign returned 404');
-			audioStore.audioUrl = url2;
+			// Use the URL from the status response, or fall back to presign.
+			const doneUrl =
+				(final as { url?: string }).url ?? (await tryPresign(slug, chapter, voice));
+			if (!doneUrl) throw new Error('Audio generated but no URL available');
+			audioStore.audioUrl = doneUrl;
 			audioStore.status = 'ready';
 			// Don't restore time for freshly generated audio — position is 0
 			// Immediately start pre-generating the next chapter in background.