feat: auto-rename album directory after in-place apply

After all file operations (rename, tag, cover, playlist), apply now
renames the album root directory to match album.json metadata:

- input_dir = CD1/CD2 etc.: parent directory is renamed automatically
  e.g. Kärntner_Doppelsextett/ → Du_Berührst_Mi_20_Jahre_Kärntner_Doppelsextett/
- input_dir = album root: a hint with the mv command is printed instead
  (avoids renaming an actively used path)
- Existing directory with target name: warning, no rename

Also: _sanitize_filename() in organizer.py made public (sanitize_filename),
used consistently across organizer, playlist and cli modules.

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

tests/test_cli.py

@@ -210,3 +210,80 @@ class TestScanCommand:
     def test_scan_no_args_fails(self) -> None:
         result = runner.invoke(app, ["scan"])
         assert result.exit_code == 1
+
+
+# ---------------------------------------------------------------------------
+# _rename_album_dir_inplace (via apply --in-place)
+# ---------------------------------------------------------------------------
+
+
+class TestRenameAlbumDir:
+    """Tests für automatisches Umbenennen des Album-Verzeichnisses."""
+
+    def test_cd_subdir_renames_parent(self, tmp_path: Path) -> None:
+        """input_dir = CD1 → Elternverzeichnis wird umbenannt."""
+        album_root = tmp_path / "Falscher_Name"
+        cd1 = album_root / "CD1"
+        cd1.mkdir(parents=True)
+
+        _make_album_json(album_root / "album.json", n_tracks=1)
+        _make_flac(cd1 / "track01.flac")
+
+        result = runner.invoke(app, [
+            "apply", str(cd1), str(album_root / "album.json"), "--in-place",
+        ])
+
+        assert result.exit_code == 0, result.output
+        expected = tmp_path / "TestAlbum_2024"
+        assert expected.exists(), f"Verzeichnis nicht umbenannt; output: {result.output}"
+        assert not album_root.exists()
+
+    def test_album_root_as_input_prints_hint(self, tmp_path: Path) -> None:
+        """input_dir = AlbumRoot → kein auto-rename, stattdessen Hinweis."""
+        album = _make_album_json(tmp_path / "album.json", n_tracks=2)
+        _make_disc_files(tmp_path, album)
+
+        result = runner.invoke(app, [
+            "apply", str(tmp_path), str(tmp_path / "album.json"), "--in-place",
+        ])
+
+        assert result.exit_code == 0, result.output
+        # Verzeichnis nicht umbenannt
+        assert tmp_path.exists()
+        # Aber Hinweis ausgegeben
+        assert "Tipp" in result.output or "mv" in result.output
+
+    def test_already_correct_name_no_rename(self, tmp_path: Path) -> None:
+        """CD1 als input_dir, Elternverzeichnis heißt bereits korrekt → keine Ausgabe."""
+        album_root = tmp_path / "TestAlbum_2024"
+        cd1 = album_root / "CD1"
+        cd1.mkdir(parents=True)
+
+        _make_album_json(album_root / "album.json", n_tracks=1)
+        _make_flac(cd1 / "track01.flac")
+
+        result = runner.invoke(app, [
+            "apply", str(cd1), str(album_root / "album.json"), "--in-place",
+        ])
+
+        assert result.exit_code == 0, result.output
+        assert album_root.exists()
+        assert "umbenannt" not in result.output
+
+    def test_target_exists_prints_warning(self, tmp_path: Path) -> None:
+        """Zielverzeichnis existiert bereits → Warnung, kein Umbenennen."""
+        album_root = tmp_path / "Falscher_Name"
+        cd1 = album_root / "CD1"
+        cd1.mkdir(parents=True)
+        # Ziel existiert bereits
+        (tmp_path / "TestAlbum_2024").mkdir()
+
+        _make_album_json(album_root / "album.json", n_tracks=1)
+        _make_flac(cd1 / "track01.flac")
+
+        result = runner.invoke(app, [
+            "apply", str(cd1), str(album_root / "album.json"), "--in-place",
+        ])
+
+        assert result.exit_code == 0, result.output
+        assert album_root.exists()  # nicht umbenannt

tests/test_organizer.py

@@ -4,36 +4,36 @@ from pathlib import Path
 
 from musiksammlung.models import Album, Disc, Track
 from musiksammlung.organizer import (
-    _sanitize_filename,
     build_mapping,
     check_disc_counts,
     discover_audio_files,
+    sanitize_filename,
 )
 
 
 class TestSanitizeFilename:
-    """Tests für _sanitize_filename."""
+    """Tests für sanitize_filename."""
 
     def test_replaces_spaces(self) -> None:
-        assert _sanitize_filename("Hello World") == "Hello_World"
+        assert sanitize_filename("Hello World") == "Hello_World"
 
     def test_replaces_punctuation(self) -> None:
-        assert _sanitize_filename("Song (Live)") == "Song_Live"
-        assert _sanitize_filename("Artist: Name") == "Artist_Name"
+        assert sanitize_filename("Song (Live)") == "Song_Live"
+        assert sanitize_filename("Artist: Name") == "Artist_Name"
 
     def test_collapses_multiple_underscores(self) -> None:
-        assert _sanitize_filename("A  B") == "A_B"
-        assert _sanitize_filename("A--B") == "A_B"
+        assert sanitize_filename("A  B") == "A_B"
+        assert sanitize_filename("A--B") == "A_B"
 
     def test_strips_leading_trailing_underscores(self) -> None:
-        assert _sanitize_filename("_Test_") == "Test"
+        assert sanitize_filename("_Test_") == "Test"
 
     def test_keeps_umlauts(self) -> None:
-        assert _sanitize_filename("Für Elise") == "Für_Elise"
-        assert _sanitize_filename("Über den Wolken") == "Über_den_Wolken"
+        assert sanitize_filename("Für Elise") == "Für_Elise"
+        assert sanitize_filename("Über den Wolken") == "Über_den_Wolken"
 
     def test_keeps_digits(self) -> None:
-        assert _sanitize_filename("Track 01") == "Track_01"
+        assert sanitize_filename("Track 01") == "Track_01"
 
 
 class TestDiscoverAudioFiles: