feat(org): add organization limit per user

Add 10 organization limit per user to prevent abuse. Includes
graceful fallback if count service is unavailable.

- Add get_user_org_count method to OrganizationService
- Check org count before allowing new organization creation
- Improve invite email mismatch error message for logged-in users

gatehouse_app/services/organization_service.py

@@ -70,6 +70,22 @@ class OrganizationService:
 
         return org
 
+    @staticmethod
+    def get_user_org_count(user_id):
+        """
+        Get the count of organizations a user belongs to.
+
+        Args:
+            user_id: User ID
+
+        Returns:
+            Count of active memberships (deleted_at is NULL)
+        """
+        return OrganizationMember.query.filter_by(
+            user_id=user_id,
+            deleted_at=None,
+        ).count()
+
     @staticmethod
     def get_organization_by_id(org_id):
         """