feat(fc3b): migration 0012 + AppSetting model

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -1,5 +1,6 @@
 """All ORM models. Import this module to make every model visible to Alembic."""
 
+from .app_setting import AppSetting
 from .artist import Artist
 from .base import Base
 from .credential import Credential
@@ -22,6 +23,7 @@ from .tag_suggestion_rejection import TagSuggestionRejection
 
 __all__ = [
     "Base",
+    "AppSetting",
     "Artist",
     "Source",
     "Credential",