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>

tests/test_organizer.py

@@ -167,8 +167,8 @@ class TestBuildMapping:
         mapping = build_mapping(album, tmp_path, tmp_path / "output")
 
         targets = list(mapping.values())
-        assert targets[0].name == "01_Erster_Song.flac"
-        assert targets[1].name == "02_Zweiter_Song.flac"
+        assert targets[0].name == "01_Erster_Song_-_TestArtist.flac"
+        assert targets[1].name == "02_Zweiter_Song_-_TestArtist.flac"
         # Single-Disc: kein CD1-Unterordner
         assert "CD1" not in str(targets[0])
 
@@ -207,7 +207,28 @@ class TestBuildMapping:
         # Source and target are in the same directory
         assert src.parent == tmp_path
         assert dst.parent == tmp_path
-        assert dst.name == "01_Song.flac"
+        assert dst.name == "01_Song_-_Artist.flac"
+
+    def test_track_artist_overrides_album_artist(self, tmp_path: Path) -> None:
+        (tmp_path / "track01.flac").touch()
+        (tmp_path / "track02.flac").touch()
+        album = Album(
+            artist="Various Artists",
+            album="Sampler",
+            discs=[
+                Disc(
+                    disc_number=1,
+                    tracks=[
+                        Track(track_number=1, title="Song A", artist="Artist X"),
+                        Track(track_number=2, title="Song B"),  # kein Track-Künstler → Fallback
+                    ],
+                )
+            ],
+        )
+        mapping = build_mapping(album, tmp_path, in_place=True)
+        targets = list(mapping.values())
+        assert targets[0].name == "01_Song_A_-_Artist_X.flac"
+        assert targets[1].name == "02_Song_B_-_Various_Artists.flac"
 
     def test_output_dir_structure(self, tmp_path: Path) -> None:
         (tmp_path / "track01.flac").touch()