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>

worker/tests/conftest.py

@@ -30,6 +30,7 @@ def conn():
         cur.execute('DELETE FROM "DiscoverySuggestion"')
         cur.execute('DELETE FROM "DiscoverySeedContribution"')
         cur.execute('DELETE FROM "ScanWorkItem"')
+        cur.execute('DELETE FROM "UnmatchedAlbum"')
         cur.execute('DELETE FROM "ApiCache"')
         cur.execute('DELETE FROM "Config"')
     connection.commit()
@@ -48,6 +49,7 @@ def conn():
         cur.execute('DELETE FROM "DiscoverySuggestion"')
         cur.execute('DELETE FROM "DiscoverySeedContribution"')
         cur.execute('DELETE FROM "ScanWorkItem"')
+        cur.execute('DELETE FROM "UnmatchedAlbum"')
         cur.execute('DELETE FROM "ApiCache"')
         cur.execute('DELETE FROM "Config"')
     connection.commit()