fix: add created_at and updated_at to _build_admin_response function

The helper function was building the response manually but was missing
the new timestamp fields added to the schema.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/api/v1/admin/admin_users.py

@@ -124,6 +124,8 @@ def _build_admin_response(admin: User) -> AdminUserResponse:
         is_active=admin.is_active,
         is_super_admin=admin.is_super_admin,
         platform_assignments=assignments,
+        created_at=admin.created_at,
+        updated_at=admin.updated_at,
     )