curl --request POST \
  --url https://upay-sistema-api.onrender.com/api/v1/coupons/validate \
  --header 'Content-Type: application/json' \
  --data '
{
  "code": "PROMO10",
  "productIds": [
    "prod_abc123"
  ],
  "amountCents": 5000
}
'

{
  "success": true,
  "data": {
    "valid": true,
    "code": "PROMO10",
    "discountType": "PERCENTAGE",
    "discountValue": 10,
    "discountAmountCents": 500,
    "finalAmountCents": 4500
  }
}