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>

web/src/app/library/library-client.tsx

@@ -16,6 +16,7 @@ type Album = {
   year: string | null;
   primaryType: string | null;
   artistMbid: string | null;
+  trackNames: string[];
 };
 
 export function LibraryClient() {
@@ -75,7 +76,7 @@ export function LibraryClient() {
         <AlbumModal
           open
           onClose={() => setOpen(null)}
-          album={{ rgMbid: open.rgMbid, album: open.album, artist: open.artist, year: open.year, type: open.primaryType, artistMbid: open.artistMbid }}
+          album={{ rgMbid: open.rgMbid, album: open.album, artist: open.artist, year: open.year, type: open.primaryType, artistMbid: open.artistMbid, trackNames: open.trackNames }}
           status={<span className="chip done">In library · {open.format}</span>}
         />
       ) : null}