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/migrations_sqlite/20260315000000_add_transcode_settings.sql

@@ -0,0 +1,5 @@
+CREATE TABLE IF NOT EXISTS transcode_settings (
+    id INTEGER PRIMARY KEY CHECK (id = 1),
+    cleanup_ttl_hours INTEGER NOT NULL DEFAULT 24
+);
+INSERT OR IGNORE INTO transcode_settings (id, cleanup_ttl_hours) VALUES (1, 24);