Harden auth flows and add backend quality gate

backend/app/services/invite_email.py

@@ -451,6 +451,10 @@ def _normalize_email(value: object) -> Optional[str]:
     return str(value).strip()
 
 
+def normalize_delivery_email(value: object) -> Optional[str]:
+    return _normalize_email(value)
+
+
 def _normalize_display_text(value: object, fallback: str = "") -> str:
     if value is None:
         return fallback
@@ -880,6 +884,9 @@ def render_invite_email_template(
 def resolve_user_delivery_email(user: Optional[Dict[str, Any]], invite: Optional[Dict[str, Any]] = None) -> Optional[str]:
     if not isinstance(user, dict):
         return _normalize_email(invite.get("recipient_email") if isinstance(invite, dict) else None)
+    stored_email = _normalize_email(user.get("email"))
+    if stored_email:
+        return stored_email
     username_email = _normalize_email(user.get("username"))
     if username_email:
         return username_email