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)