feat(transcoding): add FFmpeg HLS transcoding support

- Introduced `TranscodeManager` for managing on-demand transcoding of local video files.
- Added configuration options for transcoding in `Config` and `LocalFilesConfig`.
- Implemented new API routes for managing transcoding settings, stats, and cache.
- Updated `LocalFilesProvider` to support transcoding capabilities.
- Created frontend components for managing transcode settings and displaying stats.
- Added database migration for transcode settings.
- Enhanced existing routes and DTOs to accommodate new transcoding features.

k-tv-backend/domain/src/ports.rs

@@ -54,6 +54,8 @@ pub struct ProviderCapabilities {
     pub streaming_protocol: StreamingProtocol,
     /// Whether `POST /files/rescan` is available.
     pub rescan: bool,
+    /// Whether on-demand FFmpeg transcoding to HLS is available.
+    pub transcode: bool,
 }
 
 // ============================================================================