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/itunes.py

@@ -0,0 +1,32 @@
+"""iTunes Search API client — free, no API key, no auth, no rate-limit registration.
+Used as a fallback when MusicBrainz + Cover Art Archive has no match/art for a release.
+"""
+import requests
+from typing import Optional, Dict
+
+ITUNES_BASE = "https://itunes.apple.com/search"
+
+
+def lookup_album(artist: str, album: str) -> Optional[Dict]:
+    """Best-effort match for an artist+album against the iTunes Store catalog."""
+    try:
+        r = requests.get(ITUNES_BASE, params={
+            "term": f"{artist} {album}", "media": "music", "entity": "album", "limit": 1,
+        }, timeout=15)
+        r.raise_for_status()
+        results = (r.json() or {}).get("results") or []
+    except Exception:
+        return None
+    if not results:
+        return None
+    res = results[0]
+    art = res.get("artworkUrl100")
+    if art:
+        # iTunes serves a small default; the same CDN path accepts arbitrary sizes.
+        art = art.replace("100x100bb", "1200x1200bb")
+    return {
+        "title": res.get("collectionName") or album,
+        "artist": res.get("artistName") or artist,
+        "year": (res.get("releaseDate") or "")[:4] or None,
+        "art_url": art,
+    }