Fix matching: add missing subtitle field, proper error logging, match-all endpoint

- MatchResult was missing subtitle field, causing AttributeError in
  _apply_match that silently killed every background match task
- Wrap _apply_match in try/except with exc_info logging so failures
  are visible in docker compose logs backend
- New POST /api/libraries/:id/match-all endpoint to trigger matching
  for all unlocked items (useful for items scanned before the fix)
- Admin UI: Match button per library next to the Scan button

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/services/matching/base.py

@@ -7,6 +7,7 @@ class MatchResult:
     source: str  # musicbrainz / open_library / google_books
     source_id: str
     title: str
+    subtitle: str | None = None
     author: str | None = None
     narrator: str | None = None
     description: str | None = None