Providers now expose a source URI (for FFmpeg) instead of a
viewer-facing stream URL. Playout Service will own transcoding.
- Add SourceUri value object (NetworkUrl | FilePath)
- Remove StreamQuality, StreamingProtocol from domain
- Simplify ProviderCapabilities (drop streaming_protocol, transcode)
- Jellyfin: return static direct stream URL
- Local files: return absolute file path
- Rename use case get_stream_url -> get_source
- Stream endpoint returns JSON SourceUri instead of 307 redirect