List users with their group assignments

{
  "success": true,
  "users": [
    {
      "id": 5,
      "name": "Jane Doe",
      "email": "jane@example.com",
      "role": "client",
      "groups": [
        {
          "id": 1,
          "name": "January 2026 Cohort"
        }
      ]
    }
  ]
}