Register a new user (first user becomes superadmin)

Example response

{
  "data": {
    "user": {
      "_id": "65f1c2a4d8b9e0123456789a",
      "firstName": "Ada",
      "lastName": "Lovelace",
      "email": "ada@example.com",
      "role": [
        "admin"
      ],
      "teamId": "65f1c2a4d8b9e01234567890",
      "createdAt": "2026-04-01T10:00:00.000Z",
      "updatedAt": "2026-04-15T14:30:00.000Z"
    },
    "token": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9..."
  }
}