feat: add Airwallex payments and multi-currency support

backend/internal/payment/fee.go

@@ -4,16 +4,18 @@ import (
 	"github.com/shopspring/decimal"
 )
 
-// CalculatePayAmount computes the total pay amount given a recharge amount and
-// fee rate (percentage). Fee = amount * feeRate / 100, rounded UP (away from zero)
-// to 2 decimal places. The returned string is formatted to exactly 2 decimal places.
-// If feeRate <= 0, the amount is returned as-is (formatted to 2 decimal places).
 func CalculatePayAmount(rechargeAmount float64, feeRate float64) string {
+	return CalculatePayAmountForCurrency(rechargeAmount, feeRate, DefaultPaymentCurrency)
+}
+
+// CalculatePayAmountForCurrency 按币种精度计算应付金额，手续费向上取整到该币种最小支付单位。
+func CalculatePayAmountForCurrency(rechargeAmount float64, feeRate float64, currency string) string {
+	fractionDigits := int32(CurrencyMaxFractionDigits(currency))
 	amount := decimal.NewFromFloat(rechargeAmount)
 	if feeRate <= 0 {
-		return amount.StringFixed(2)
+		return amount.StringFixed(fractionDigits)
 	}
 	rate := decimal.NewFromFloat(feeRate)
-	fee := amount.Mul(rate).Div(decimal.NewFromInt(100)).RoundUp(2)
-	return amount.Add(fee).StringFixed(2)
+	fee := amount.Mul(rate).Div(decimal.NewFromInt(100)).RoundUp(fractionDigits)
+	return amount.Add(fee).StringFixed(fractionDigits)
 }