Sanitize filenames: replace spaces/punctuation with underscores

Replace all non-word characters (spaces, punctuation, brackets, etc.)
with underscores in track titles, album names and artist names.
Collapse consecutive underscores to one, strip leading/trailing ones.
Umlauts (ä, ö, ü, ß) and digits are preserved.

Also use underscore instead of space between track number and title.

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

src/musiksammlung/organizer.py

@@ -38,8 +38,14 @@ class DiscCheck:
 
 
 def _sanitize_filename(name: str) -> str:
-    """Entfernt problematische Zeichen aus Dateinamen."""
-    return re.sub(r'[<>:"/\\|?*]', "_", name).strip()
+    """Ersetzt Sonderzeichen und Leerzeichen durch Unterstriche.
+
+    Buchstaben (inkl. Umlaute), Ziffern und bestehende Unterstriche bleiben erhalten.
+    Mehrere aufeinanderfolgende Unterstriche werden zu einem zusammengezogen.
+    """
+    name = re.sub(r'[^\w]', '_', name)   # alles außer Buchstaben/Ziffern/_  → _
+    name = re.sub(r'_+', '_', name)       # mehrere _ → einer
+    return name.strip('_')
 
 
 def discover_audio_files(directory: Path) -> list[Path]:
@@ -118,7 +124,7 @@ def build_mapping(
 
         for audio_file, track in zip(audio_files, disc.tracks):
             safe_title = _sanitize_filename(track.title)
-            new_name = f"{track.track_number:02d} {safe_title}{audio_file.suffix}"
+            new_name = f"{track.track_number:02d}_{safe_title}{audio_file.suffix}"
             mapping[audio_file] = target_dir / new_name
 
     return mapping