feat(library): surface albums the scan couldn't match

The library scan silently skipped album folders with no MusicBrainz match
(or an unreadable file). Persist them so they're visible and fixable.

- New UnmatchedAlbum model (migration add_unmatched_album), keyed by path.
- scan_chunk records a skipped album as unmatched (reason "no MusicBrainz
  match" or "unreadable: <err>"), clears the row when an album later
  resolves, and prunes stale rows (a prior scan's, or a removed folder)
  when a scan completes.
- GET /api/library/unmatched lists them; DELETE ?id= dismisses one.
- /library shows a "Couldn't match" section (path + reason + Dismiss).

Worker + web tests added.

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

web/src/test/setup.ts

@@ -18,4 +18,5 @@ beforeEach(async () => {
   await prisma.discoverySuggestion.deleteMany();
   await prisma.config.deleteMany();
   await prisma.apiCache.deleteMany(); // read-through cache must not leak across tests
+  await prisma.unmatchedAlbum.deleteMany();
 });