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/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 ─────────────────────────────────────────────────────