Add per-track artist to filename: 09_Titel_-_Kuenstler.flac

- 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>

src/musiksammlung/playlist.py

@@ -35,7 +35,7 @@ def generate_playlist(album: Album, album_dir: Path) -> Path:
         for track in disc.tracks:
             safe_title = _sanitize_filename(track.title)
             # Audiodatei im Zielverzeichnis finden
-            pattern = f"{track.track_number:02d}_{safe_title}.*"
+            pattern = f"{track.track_number:02d}_{safe_title}*"
             if multi_disc:
                 search_dir = album_dir / f"CD{disc.disc_number}"
             else: