curl -X POST "https://api.nevatal.com/api/v1/appointments/apt_xyz789/cancel" \
  -H "Authorization: Bearer nvtl_your_api_key" \
  -H "Content-Type: application/json" \
  -d '{ "reason": "Patient requested reschedule" }'

{
  "id": "apt_xyz789",
  "status": "canceled",
  "canceledAt": "2025-03-21T08:30:00Z",
  "cancelReason": "Patient requested reschedule"
}

{
  "statusCode": 422,
  "error": "Unprocessable Entity",
  "message": "Cannot cancel a completed appointment"
}