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>

src/musiksammlung/playlist.py

@@ -6,7 +6,7 @@ import logging
 from pathlib import Path
 
 from musiksammlung.models import Album
-from musiksammlung.organizer import _sanitize_filename
+from musiksammlung.organizer import sanitize_filename
 
 logger = logging.getLogger(__name__)
 
@@ -20,7 +20,7 @@ def generate_playlist(album: Album, album_dir: Path) -> Path:
     Returns:
         Pfad zur erzeugten Playlist-Datei.
     """
-    playlist_name = _sanitize_filename(album.album) + ".m3u"
+    playlist_name = sanitize_filename(album.album) + ".m3u"
     playlist_path = album_dir / playlist_name
     multi_disc = len(album.discs) > 1
 
@@ -33,7 +33,7 @@ def generate_playlist(album: Album, album_dir: Path) -> Path:
             disc_prefix = ""
 
         for track in disc.tracks:
-            safe_title = _sanitize_filename(track.title)
+            safe_title = sanitize_filename(track.title)
             # Audiodatei im Zielverzeichnis finden
             pattern = f"{track.track_number:02d}_-_{safe_title}*"
             if multi_disc: