curl --request GET \
  --url https://api.railmail.app/api/v1/billing/subscription \
  --header 'X-API-Key: <api-key>'

{
  "id": 123,
  "planTier": "<string>",
  "status": "<string>",
  "billingInterval": "<string>",
  "aiCreditsTier": 123,
  "pendingPlanTier": "<string>",
  "pendingAiCreditsTier": 123,
  "pendingPlanEffectiveAt": "2023-11-07T05:31:56Z",
  "currentPeriodStart": "2023-11-07T05:31:56Z",
  "currentPeriodEnd": "2023-11-07T05:31:56Z",
  "cancelledAt": "2023-11-07T05:31:56Z",
  "cancellationReason": "<string>",
  "cancelAtPeriodEnd": true,
  "isSendingAllowed": true,
  "limits": {
    "maxSubscribers": 123,
    "maxCampaignsPerMonth": 123,
    "maxProjects": 123,
    "maxAiRequestsPerMonth": 123,
    "maxUsers": 123,
    "maxEmailsPerMonth": 123,
    "customFieldsEnabled": true,
    "automationsEnabled": true,
    "advancedSegmentsEnabled": true,
    "customDomainEnabled": true,
    "apiAccessEnabled": true,
    "prioritySupport": true
  },
  "createdAt": "2023-11-07T05:31:56Z",
  "updatedAt": "2023-11-07T05:31:56Z"
}