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
0d7b985469