- Track model: add optional artist field (None = fall back to album artist)
- organizer: append _-_<artist> to each filename
- tagger: use track.artist over album.artist for the 'artist' tag
- playlist: widen glob pattern to match new _-_<artist> suffix
- tests: update assertions + add test for track-artist override
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>