Checkout de assinatura

POST /api/subscriptions/checkout

curl --request POST \
  --url https://upay-sistema-api.onrender.com/api/subscriptions/checkout \
  --header 'Content-Type: application/json' \
  --data '{
    "planId": "550e8400-e29b-41d4-a716-446655440000",
    "clientName": "João Silva",
    "clientEmail": "joao@example.com",
    "clientDocument": "12345678901",
    "paymentMethod": "CREDIT_CARD",
    "cardToken": "tok_abc123"
  }'

{
  "success": true,
  "data": {
    "subscriptionId": "a0eebc99-9c0b-4ef8-bb6d-6bb9bd380a11",
    "status": "TRIAL",
    "nextBillingDate": "2026-06-01T00:00:00.000Z"
  }
}