feat: show real on-disk tracklist in the Library album modal (#9)

The Library modal showed MusicBrainz's canonical tracklist, hiding incomplete
downloads / edition or track-order mismatches, and showed nothing for albums
without an rgMbid. Now it shows the REAL files on disk.

- LibraryItem gains trackNames String[] (migration add_library_track_names):
  the album's on-disk "## Title.ext" audio filenames.
- Captured at import (pipeline._import lists the final album folder) and at scan
  (scan._record_owned now also refreshes trackNames on re-scan via ON CONFLICT
  DO UPDATE + xmax=0 to preserve the newly-imported flag). New shared
  library.list_audio_files() helper.
- /api/library exposes trackNames; the AlbumModal prefers on-disk tracks when
  present (parsed "## Title" → position/title, zero network, labeled "On disk ·
  N tracks"), falling back to the MusicBrainz listing otherwise. Items imported
  before this column show the MB fallback until re-scanned.

worker 221 tests / 7-skip, web 153, tsc + build clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

worker/tests/test_list_audio_files.py

@@ -0,0 +1,20 @@
+from lyra_worker.library import list_audio_files
+
+
+def test_lists_sorted_audio_basenames_only(tmp_path):
+    (tmp_path / "02 Two.flac").write_bytes(b"")
+    (tmp_path / "01 One.flac").write_bytes(b"")
+    (tmp_path / "10 Ten.mp3").write_bytes(b"")
+    (tmp_path / "cover.jpg").write_bytes(b"")       # non-audio excluded
+    (tmp_path / "notes.txt").write_bytes(b"")       # non-audio excluded
+    (tmp_path / "Disc 1").mkdir()                    # a subdir is not a file
+
+    assert list_audio_files(str(tmp_path)) == ["01 One.flac", "02 Two.flac", "10 Ten.mp3"]
+
+
+def test_missing_dir_returns_empty():
+    assert list_audio_files("/no/such/album/dir") == []
+
+
+def test_empty_dir_returns_empty(tmp_path):
+    assert list_audio_files(str(tmp_path)) == []