feat(api): return 403 when attempting to remove last owner

Handle edge case where removing a member would leave an organization
without any owners. Service layer raises ValueError for this scenario,
which the API endpoint catches and converts to a forbidden response
with actionable error message about transferring ownership.

gatehouse_app/api/v1/organizations/members.py

@@ -56,7 +56,10 @@ def add_organization_member(org_id):
 @full_access_required
 def remove_organization_member(org_id, user_id):
     org = OrganizationService.get_organization_by_id(org_id)
-    OrganizationService.remove_member(org=org, user_id=user_id, remover_id=g.current_user.id)
+    try:
+        OrganizationService.remove_member(org=org, user_id=user_id, remover_id=g.current_user.id)
+    except ValueError as e:
+        return api_response(success=False, message=str(e), status=403, error_type="OWNER_PROTECTION")
     return api_response(message="Member removed successfully")