fix(audio): return 404 from presign when audio object not yet uploaded

handlePresignAudio now checks AudioExists before presigning; previously it
would generate a valid-looking signed URL for a non-existent object, causing
the browser to get 404 from MinIO directly.

- Add AudioExists to Store interface and HybridStore
- Guard handlePresignAudio with AudioExists check, return 404 if missing
- Propagate 404 through presignAudio() in minio.ts (typed error.status=404)
- SvelteKit presign route forwards 404 to the browser instead of 500

scraper/internal/server/server.go

@@ -719,6 +719,14 @@ func (s *Server) handlePresignAudio(w http.ResponseWriter, r *http.Request) {
 	}
 
 	key := s.store.AudioObjectKey(slug, n, voice, speed)
+
+	// Return 404 when the object hasn't been uploaded yet — the client treats
+	// this as "audio not ready" and will either poll or trigger generation.
+	if !s.store.AudioExists(r.Context(), key) {
+		http.NotFound(w, r)
+		return
+	}
+
 	url, err := s.store.PresignAudio(r.Context(), key, 1*time.Hour)
 	if err != nil {
 		s.log.Error("presign audio failed", "slug", slug, "n", n, "err", err)

scraper/internal/storage/hybrid.go

@@ -291,6 +291,10 @@ func (h *HybridStore) AudioObjectKey(slug string, n int, voice string, speed flo
 	return AudioObjectKey(slug, n, voice, speed)
 }
 
+func (h *HybridStore) AudioExists(ctx context.Context, key string) bool {
+	return h.minio.AudioExists(ctx, key)
+}
+
 // ─── PutAudio ─────────────────────────────────────────────────────────────────
 
 func (h *HybridStore) PutAudio(ctx context.Context, key string, data []byte) error {

scraper/internal/storage/store.go

@@ -130,6 +130,8 @@ type Store interface {
 
 	// AudioObjectKey returns the MinIO object key for a cached audio file.
 	AudioObjectKey(slug string, n int, voice string, speed float64) string
+	// AudioExists returns true when the audio object is present in the bucket.
+	AudioExists(ctx context.Context, key string) bool
 
 	// ── Presigned URLs ─────────────────────────────────────────────────────
 

ui/src/lib/server/minio.ts

@@ -64,6 +64,7 @@ export async function presignChapter(slug: string, n: number, rewrite = false):
 /**
  * Returns a presigned URL for an audio file.
  * URL is valid for ~1 hour. The URL is returned to the browser for direct streaming.
+ * Throws with { status: 404 } when the audio object has not been generated yet.
  */
 export async function presignAudio(
 	slug: string,
@@ -83,6 +84,12 @@ export async function presignAudio(
 		log.error('minio', 'presign audio network error', { slug, n, err: String(e) });
 		throw new Error(`presign audio ${slug}/${n}: network error`);
 	}
+	if (res.status === 404) {
+		// Audio hasn't been generated / uploaded yet — caller should surface this as 404.
+		const err = new Error(`presign audio ${slug}/${n}: not found`) as Error & { status: number };
+		err.status = 404;
+		throw err;
+	}
 	if (!res.ok) {
 		const body = await res.text().catch(() => '');
 		log.error('minio', 'presign audio failed', { slug, n, status: res.status, body });

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

@@ -7,6 +7,7 @@ import { log } from '$lib/server/logger';
  * GET /api/presign/audio?slug=...&n=...&voice=...&speed=...
  * Returns a presigned MinIO URL for the audio file so the browser
  * can stream it directly without going through the server.
+ * Returns 404 when the audio has not been generated yet.
  */
 export const GET: RequestHandler = async ({ url }) => {
 	const slug = url.searchParams.get('slug');
@@ -22,6 +23,10 @@ export const GET: RequestHandler = async ({ url }) => {
 		const presignedUrl = await presignAudio(slug, n, voice, speed);
 		return json({ url: presignedUrl });
 	} catch (e) {
+		const status = (e as { status?: number }).status;
+		if (status === 404) {
+			error(404, 'Audio not found');
+		}
 		log.error('presign', 'presign audio failed', { slug, n, err: String(e) });
 		error(500, `Could not get presigned URL: ${e}`);
 	}