feat(auth): add oauth_identities table for Google/Microsoft sign-in

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

backend/app/models/__init__.py

@@ -62,6 +62,7 @@ from .session_fact import SessionFact
 from .session_suggested_fix import SessionSuggestedFix
 from .draft_template import DraftTemplate
 from .account_settings import AccountSettings
+from .oauth_identity import OAuthIdentity  # noqa: F401
 
 __all__ = [
     "User",
@@ -138,4 +139,5 @@ __all__ = [
     "SessionSuggestedFix",
     "DraftTemplate",
     "AccountSettings",
+    "OAuthIdentity",
 ]