Case-insensitive login, auto-matching after scan, per-library sources

- auth: username lookup is now case-insensitive via func.lower()
- scanner: trigger match_audiobook for each newly found item after scan
- matcher: read match_sources from library settings; refactored to loop
  over configured sources in priority order instead of hardcoded sequence
- schemas/routers: expose matchSources in LibraryOut API response
- Admin UI: pill-toggle for MusicBrainz/Open Library/Google Books per library

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

backend/app/routers/libraries.py

@@ -32,6 +32,7 @@ def _library_to_out(lib: Library) -> dict:
         media_type=lib.media_type,
         icon=settings_data.get("icon", "database"),
         provider=settings_data.get("provider", "google"),
+        match_sources=settings_data.get("match_sources", ["musicbrainz", "open_library", "google_books"]),
         created_at=int(lib.created_at.timestamp() * 1000) if lib.created_at else 0,
         last_update=int(lib.updated_at.timestamp() * 1000) if lib.updated_at else 0,
     )