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.

frontend/src/components/Navbar.jsx

@@ -32,6 +32,7 @@ export const Navbar = () => {
               <NavLink to="/browse" className={linkClass} data-testid="nav-browse">Library</NavLink>
               <NavLink to="/shows" className={linkClass} data-testid="nav-shows">Series</NavLink>
               <NavLink to="/my-list" className={linkClass} data-testid="nav-my-list">Shelf</NavLink>
+              <NavLink to="/music" className={linkClass} data-testid="nav-music">Music</NavLink>
               <NavLink to="/requests" className={linkClass} data-testid="nav-requests">Wishlist</NavLink>
               {user.is_admin && <NavLink to="/admin" className={linkClass} data-testid="nav-admin">Admin</NavLink>}
               {user.is_admin && <NavLink to="/admin/users" className={linkClass} data-testid="nav-users">Users</NavLink>}