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/musicbrainz.py

@@ -0,0 +1,42 @@
+"""MusicBrainz + Cover Art Archive clients — both free, no API key.
+MusicBrainz rate limit is 1 req/sec for unauthenticated clients; callers must pace requests.
+"""
+import requests
+from typing import Optional, Dict
+
+_HEADERS = {"User-Agent": "StreamHoard/1.0 (self-hosted media library)"}
+MB_BASE = "https://musicbrainz.org/ws/2"
+COVER_ART_BASE = "https://coverartarchive.org"
+
+
+def lookup_release(artist: str, album: str) -> Optional[Dict]:
+    """Best-effort match for an artist+album against MusicBrainz's release index."""
+    query = f'release:"{album}" AND artist:"{artist}"'
+    try:
+        r = requests.get(f"{MB_BASE}/release/", params={"query": query, "fmt": "json", "limit": 1},
+                          headers=_HEADERS, timeout=15)
+        r.raise_for_status()
+        releases = (r.json() or {}).get("releases") or []
+    except Exception:
+        return None
+    if not releases:
+        return None
+    rel = releases[0]
+    return {
+        "mbid": rel.get("id"),
+        "title": rel.get("title") or album,
+        "artist": (rel.get("artist-credit") or [{}])[0].get("name") or artist,
+        "year": (rel.get("date") or "")[:4] or None,
+    }
+
+
+def cover_art_url(mbid: str) -> Optional[str]:
+    """Returns the front cover URL if the Cover Art Archive has one for this release, else None."""
+    url = f"{COVER_ART_BASE}/release/{mbid}/front"
+    try:
+        r = requests.head(url, headers=_HEADERS, timeout=10, allow_redirects=True)
+        if r.status_code == 200:
+            return r.url
+    except Exception:
+        pass
+    return None