feat: add admin user serializer with accept invitation url

packages/backend/src/serializers/index.js

@@ -16,8 +16,10 @@ import actionSerializer from './action.js';
 import executionSerializer from './execution.js';
 import executionStepSerializer from './execution-step.js';
 import subscriptionSerializer from './subscription.ee.js';
+import adminUserSerializer from './admin/user.js';
 
 const serializers = {
+  AdminUser: adminUserSerializer,
   User: userSerializer,
   Role: roleSerializer,
   Permission: permissionSerializer,