curl --request GET \
  --url https://end-api.shallow.ink/api/v1/subscription/checkin/challenge \
  --header 'X-API-Key: <api-key>'

{
  "code": 0,
  "data": {
    "challenge_id": "<string>",
    "timestamp": 123,
    "server_nonce": "<string>",
    "sign_token": "<string>"
  }
}