Speed up music scan by deferring tag reads to import time

Reading embedded ID3/MP4 tags for every file during the scan itself was
taking minutes even for a small (146-track) library, since each mutagen
open is a slow round trip over the CIFS-mounted NAS share. Scan now uses
fast filename/path parsing only; tag reads happen once, only for the
tracks actually selected for import.

backend/library_scan.py

@@ -132,7 +132,10 @@ def _read_tags(path: Path) -> Dict:
 
 
 def scan_music() -> List[Dict]:
-    """Music/<Artist>/<Album>/<NN Title.ext>. Prefers embedded tags, falls back to path/filename."""
+    """Music/<Artist>/<Album>/<NN Title.ext>. Filename/path-derived only — no file reads, so this
+    stays fast even over a network share with a large library. Embedded tags (better titles, real
+    duration) are read later, only for the tracks actually selected for import — see tags_for_file().
+    """
     if not MUSIC_DIR.is_dir():
         return []
     out = []
@@ -145,23 +148,24 @@ def scan_music() -> List[Dict]:
             for f in sorted(album_dir.iterdir()):
                 if not f.is_file() or f.suffix.lower() not in AUDIO_EXTS:
                     continue
-                tags = _read_tags(f)
-                track_number = tags["track_number"]
-                title = tags["title"]
-                if not title:
-                    fm = _TRACK_NUM_RE.match(f.stem)
-                    if fm:
-                        track_number = track_number or int(fm.group(1))
-                        title = fm.group(2)
-                    else:
-                        title = f.stem
+                track_number = None
+                title = f.stem
+                fm = _TRACK_NUM_RE.match(f.stem)
+                if fm:
+                    track_number = int(fm.group(1))
+                    title = fm.group(2)
                 out.append({
-                    "artist": tags["artist"] or artist_dir.name,
-                    "album": tags["album"] or album_dir.name,
+                    "artist": artist_dir.name,
+                    "album": album_dir.name,
                     "title": _clean_title(title) or title,
                     "track_number": track_number,
-                    "duration_seconds": round(tags["duration"], 1),
+                    "duration_seconds": 0,
                     "storage_path": str(f),
                     "size_bytes": f.stat().st_size,
                 })
     return out
+
+
+def tags_for_file(path_str: str) -> Dict:
+    """Embedded-tag lookup for a single file — used at import time, not during scan."""
+    return _read_tags(Path(path_str))