Add iTunes and AcoustID as additional free music enrichment sources

- itunes.py: zero-auth fallback for album artwork when MusicBrainz/Cover Art
  Archive has no match — kicks in automatically during music enrichment,
  toggleable in Settings (on by default, no key required).
- acoustid.py: audio fingerprint identification (Chromaprint fpcalc + the
  AcoustID API) for tracks with missing/unknown artist or album — nothing
  to text-search on, so this identifies from the actual audio content
  instead. Requires a free user-supplied API key from acoustid.org/api-key,
  off by default. New /music/identify endpoint + admin trigger button.
- Dockerfile: added libchromaprint-tools for fpcalc.
- Settings page: new Music Metadata & Artwork section with both toggles.

backend/models.py

@@ -225,6 +225,9 @@ class AppSettings(BaseModel):
     trakt_client_secret: str = ""
     auto_transcode: str = "off"
     queue_paused: bool = False
+    music_itunes_fallback: bool = True
+    music_acoustid_enabled: bool = False
+    acoustid_api_key: str = ""
 
 
 class AppSettingsPublic(BaseModel):
@@ -243,6 +246,10 @@ class AppSettingsPublic(BaseModel):
     trakt_client_secret: str = ""
     auto_transcode: str = "off"
     queue_paused: bool = False
+    music_itunes_fallback: bool = True
+    music_acoustid_enabled: bool = False
+    acoustid_api_key: str = ""
+    acoustid_configured: bool = False
 
 
 # ---------- Shows / Episodes (Sonarr) ----------