curl -X POST "https://demo.api.vultlocal.com/api/v1/subscribers/sub_abc123/change-pin" \
  -H "Authorization: Bearer olive_live_xxx" \
  -H "Content-Type: application/json" \
  -d '{
    "current_pin": "1234",
    "new_pin": "5678"
  }'

{
  "success": true,
  "message": "PIN changed successfully"
}

{
  "success": false,
  "message": "Current PIN is incorrect",
  "attempts_remaining": 2
}

{
  "success": false,
  "message": "New PIN must be 4 digits",
  "code": "INVALID_PIN_FORMAT"
}