Add Discogs as cover source fallback after MusicBrainz

- _discogs_cover_url(): searches Discogs database/search API by artist+album,
  returns primary image URL; uses DISCOGS_TOKEN if set, else anonymous
- download_discogs_cover(): downloads and saves as folder.jpg (PNG→JPEG via PIL)
- resolve_cover() priority: local → MusicBrainz → Discogs
- music_enricher: pass artist/album to resolve_cover() for Discogs lookup

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

music_enricher.py

@@ -148,6 +148,8 @@ def process_album(
             hints.cover_images,
             proposal.mbid,
             album_dir,
+            artist=proposal.albumartist,
+            album=proposal.album,
         )
         if cover_path and not args.no_cover:
             proposal.cover_path = cover_path