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/prisma/schema.prisma

@@ -82,6 +82,11 @@ model LibraryItem {
   format       String
   qualityClass Int
   importedAt   DateTime @default(now())
+  // The album's on-disk audio filenames ("## Title.ext"), captured at import + scan, so the
+  // Library modal can show the REAL tracks on disk (revealing incomplete/edition mismatches)
+  // instead of MusicBrainz's canonical listing. Empty for items imported before this existed
+  // (re-scan to populate).
+  trackNames   String[] @default([])
 
   @@unique([artist, album])
 }