Cover-Embedding: frontcover.jpg/backcover.jpg als Standard-Konvention

cover: find_cover() sucht frontcover.jpg/.png und backcover.jpg/.png;
  copy_covers() speichert als frontcover.jpg / backcover.jpg
tagger: embed_album_cover() bettet Frontcover in alle Tracks ein
cli: apply und process rufen embed_album_cover() nach copy_covers() auf
tests: TestFindCover mit 7 Tests (jpg, png, Symlink, Priorität, Negativ)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

tests/test_cover.py

@@ -0,0 +1,45 @@
+"""Tests für Cover-Funktionen."""
+
+from pathlib import Path
+
+import pytest
+
+from musiksammlung.cover import find_cover
+
+
+class TestFindCover:
+    """Tests für find_cover."""
+
+    def test_finds_frontcover_jpg(self, tmp_path: Path) -> None:
+        (tmp_path / "frontcover.jpg").touch()
+        assert find_cover(tmp_path, "front") == tmp_path / "frontcover.jpg"
+
+    def test_finds_frontcover_png(self, tmp_path: Path) -> None:
+        (tmp_path / "frontcover.png").touch()
+        assert find_cover(tmp_path, "front") == tmp_path / "frontcover.png"
+
+    def test_jpg_preferred_over_png(self, tmp_path: Path) -> None:
+        (tmp_path / "frontcover.jpg").touch()
+        (tmp_path / "frontcover.png").touch()
+        # .jpg wird zuerst geprüft
+        assert find_cover(tmp_path, "front") == tmp_path / "frontcover.jpg"
+
+    def test_finds_backcover(self, tmp_path: Path) -> None:
+        (tmp_path / "backcover.jpg").touch()
+        assert find_cover(tmp_path, "back") == tmp_path / "backcover.jpg"
+
+    def test_returns_none_if_missing(self, tmp_path: Path) -> None:
+        assert find_cover(tmp_path, "front") is None
+        assert find_cover(tmp_path, "back") is None
+
+    def test_follows_symlink(self, tmp_path: Path) -> None:
+        real = tmp_path / "original.jpg"
+        real.touch()
+        link = tmp_path / "frontcover.jpg"
+        link.symlink_to(real)
+        assert find_cover(tmp_path, "front") == link
+
+    def test_ignores_wrong_names(self, tmp_path: Path) -> None:
+        (tmp_path / "cover.jpg").touch()
+        (tmp_path / "back.jpg").touch()
+        assert find_cover(tmp_path, "front") is None