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/amount.go

@@ -0,0 +1,24 @@
+package payment
+
+import (
+	"fmt"
+
+	"github.com/shopspring/decimal"
+)
+
+const centsPerYuan = 100
+
+// YuanToFen converts a CNY yuan string (e.g. "10.50") to fen (int64).
+// Uses shopspring/decimal for precision.
+func YuanToFen(yuanStr string) (int64, error) {
+	d, err := decimal.NewFromString(yuanStr)
+	if err != nil {
+		return 0, fmt.Errorf("invalid amount: %s", yuanStr)
+	}
+	return d.Mul(decimal.NewFromInt(centsPerYuan)).IntPart(), nil
+}
+
+// FenToYuan converts fen (int64) to yuan as a float64 for interface compatibility.
+func FenToYuan(fen int64) float64 {
+	return decimal.NewFromInt(fen).Div(decimal.NewFromInt(centsPerYuan)).InexactFloat64()
+}