Add filesystem Library Scan (movies/tv/music) and a full Music library

- New admin Library Scan page walks the NAS mounts directly for movies,
  TV, and music not already in Radarr/Sonarr/local storage, and imports
  them with storage_type=scan (fully integrated: streamable, transcodable,
  listed in Admin) — Radarr/Sonarr import flows are untouched.
- New Music section (nav tab, album-grid browse, per-album track list,
  persistent bottom player with queue/seek) backed by a Track model and
  range-request audio streaming endpoint.
- MusicBrainz + Cover Art Archive integration (free, no API key) to fill
  in album art for scanned tracks, paced to their 1 req/sec rate limit.
- Mounted the NAS music share into CT104 (new mp2 LXC mountpoint,
  CIFS read-only) alongside the existing video mount.
- Optional Bluetooth/output-device picker on the player bar via the Audio
  Output Devices API (setSinkId) for already-paired speakers — Chrome/Edge
  only, since browsers can't pair new BT devices directly.

backend/requirements.txt

@@ -25,3 +25,4 @@ python-multipart>=0.0.9
 jq>=1.6.0
 typer>=0.9.0
 docker>=7.1.0
+mutagen>=1.47.0