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_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: