feat(payment): add complete payment system with multi-provider support

Add a full payment and subscription system supporting EasyPay (Alipay/WeChat),
Stripe, and direct Alipay/WeChat Pay providers with multi-instance load balancing.

backend/internal/payment/fee.go

@@ -0,0 +1,19 @@
+package payment
+
+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 {
+	amount := decimal.NewFromFloat(rechargeAmount)
+	if feeRate <= 0 {
+		return amount.StringFixed(2)
+	}
+	rate := decimal.NewFromFloat(feeRate)
+	fee := amount.Mul(rate).Div(decimal.NewFromInt(100)).RoundUp(2)
+	return amount.Add(fee).StringFixed(2)
+}