Add 4 new cover/tracklist sources: MB back cover, iTunes, Last.fm, Discogs tracklist

cover_handler.py:
- _download_image(): shared helper replaces duplicated download logic
- download_back_cover(): fetches back cover from MusicBrainz CAA (/back endpoint),
  saves as back.jpg; skips if already present
- _itunes_cover_url() / download_itunes_cover(): iTunes Search API (no auth),
  requests 600x600 artwork; fallback after Discogs
- _lastfm_cover_url() / download_lastfm_cover(): Last.fm album.getinfo
  (LASTFM_API_KEY env var); last cover fallback, skips placeholder images
- resolve_cover(): extended with iTunes → Last.fm fallback chain

metadata_resolver.py:
- _discogs_get_tracklist(): fetches full Discogs release via REST API,
  parses tracklist[] including heading-based disc detection
- _lastfm_tracklist(): fetches Last.fm album.getinfo tracks (LASTFM_API_KEY)
- resolve(): uses Discogs tracklist → Last.fm tracklist as fallback when
  MusicBrainz returns no tracks; LASTFM_API_KEY added to env var block

music_enricher.py:
- process_album(): calls download_back_cover() after execute_album() when MBID known

New cover priority:  local → MusicBrainz front → Discogs → iTunes → Last.fm
New tracklist priority: local → YouTube → MusicBrainz → Discogs → Last.fm → OCR
Test suite: 29 → 33 tests (all pass)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

music_enricher.py

@@ -26,7 +26,7 @@ from models import AlbumProposal
 from scanner import scan_album, collect_album_dirs
 from hint_extractor import extract_hints
 from metadata_resolver import resolve
-from cover_handler import resolve_cover
+from cover_handler import resolve_cover, download_back_cover
 from executor import execute_album, write_report
 
 
@@ -216,6 +216,12 @@ def process_album(
         for k, v in album_stats.items():
             stats[k] = stats.get(k, 0) + v
 
+        # Back-Cover von MusicBrainz holen (wenn MBID bekannt und noch kein back.jpg)
+        if proposal.mbid and not args.no_cover and not args.dry_run:
+            back = download_back_cover(proposal.mbid, album_dir)
+            if back:
+                print(f"  🖼️  Back-Cover heruntergeladen: {back.name}")
+
         # Jellyfin Playlist Generator aufrufen
         generator_path = _find_jellyfin_generator(album_dir, getattr(args, "playlist_generator", None))
         if generator_path: