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/lyra_worker/scan.py

@@ -5,6 +5,7 @@ from dataclasses import dataclass
 
 import psycopg
 
+from lyra_worker.library import list_audio_files
 from lyra_worker.probe import AudioProbe
 
 _AUDIO_EXT = {".flac", ".mp3", ".m4a", ".opus", ".ogg", ".aac", ".wav"}
@@ -80,14 +81,18 @@ def _record_owned(conn, target, quality_class: int, fmt: str, path: str, watched
                 '  "watchedArtistId" = COALESCE("MonitoredRelease"."watchedArtistId", EXCLUDED."watchedArtistId")',
                 (watched_id, target.artist_mbid, target.artist, target.rg_mbid, target.album, quality_class),
             )
+        track_names = list_audio_files(path)  # capture the real on-disk tracks
         cur.execute(
             'INSERT INTO "LibraryItem" (id, "requestId", artist, album, path, source, format, '
-            '"qualityClass", "importedAt") '
-            "VALUES (gen_random_uuid()::text, NULL, %s, %s, %s, 'scan', %s, %s, now()) "
-            'ON CONFLICT (artist, album) DO NOTHING',
-            (target.artist, target.album, path, fmt, quality_class),
+            '"qualityClass", "trackNames", "importedAt") '
+            "VALUES (gen_random_uuid()::text, NULL, %s, %s, %s, 'scan', %s, %s, %s, now()) "
+            # refresh trackNames on re-scan (populates items imported before this column);
+            # xmax=0 ⇒ this was an INSERT (a genuinely new library item) vs an UPDATE.
+            'ON CONFLICT (artist, album) DO UPDATE SET "trackNames" = EXCLUDED."trackNames" '
+            "RETURNING (xmax = 0) AS inserted",
+            (target.artist, target.album, path, fmt, quality_class, track_names),
         )
-        newly = cur.rowcount == 1  # a new LibraryItem means this album wasn't recorded before
+        newly = cur.fetchone()[0]  # a new LibraryItem means this album wasn't recorded before
     conn.commit()
     return newly