- Add HLSEnabled and HLSStorageDir to backend config (HLS_STREAMING env)
- Register HLS serving routes (master.m3u8, quality playlist, segments)
behind HLSEnabled feature flag on existing track routes
- Add GetHLSStatus and TriggerHLSTranscode methods to StreamService
for stream server communication
- Update docker-compose (dev, staging, prod) with HLS env vars and
shared hls-data volume between backend and stream-server
- Stream callback already correctly updates stream_manifest_url
