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/prisma/schema.prisma

@@ -191,6 +191,21 @@ model ScanWorkItem {
   @@index([scanId, done, artist, album])
 }
 
+// Album folders on disk that the library scan could not resolve to a MusicBrainz release
+// (no match, or an unreadable/corrupt file). Surfaced on /library so the user can fix the
+// metadata or add them manually instead of them being silently skipped. Keyed by path; a
+// row is removed when the album later matches, and stale rows (a prior scan's, or a folder
+// since removed) are pruned when a scan completes.
+model UnmatchedAlbum {
+  id        String   @id @default(cuid())
+  artist    String
+  album     String
+  path      String   @unique
+  reason    String
+  scanId    String
+  scannedAt DateTime @default(now())
+}
+
 // Generic read-through cache for external API responses shared by web + worker (keyed on
 // the LOGICAL operation, not the request URL, so both processes share entries). TTL is
 // applied at read time from fetchedAt in code (no expiresAt column → tunable, no migration).