feat(billing): add plan_billing sibling table for Stripe + catalog metadata

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -63,6 +63,7 @@ from .session_suggested_fix import SessionSuggestedFix
 from .draft_template import DraftTemplate
 from .account_settings import AccountSettings
 from .oauth_identity import OAuthIdentity  # noqa: F401
+from .plan_billing import PlanBilling  # noqa: F401
 
 __all__ = [
     "User",
@@ -140,4 +141,5 @@ __all__ = [
     "DraftTemplate",
     "AccountSettings",
     "OAuthIdentity",
+    "PlanBilling",
 ]