fix: 修复BepUsdt签名问题

- 将Amount字段改为string类型确保签名和请求格式一致
- 使用格式化的金额字符串进行签名和请求
- 增强钱包地址获取逻辑，支持多个字段名

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/api/handlers/payment.go

@@ -28,7 +28,7 @@ func NewPaymentHandler() *PaymentHandler {
 type BepUsdtCreateRequest struct {
 	TradeType   string  `json:"trade_type,omitempty"`
 	OrderID     string  `json:"order_id"`
-	Amount      float64 `json:"amount"`
+	Amount      string `json:"amount"`
 	NotifyURL   string  `json:"notify_url"`
 	RedirectURL string  `json:"redirect_url,omitempty"`
 	Signature   string  `json:"signature"`
@@ -90,9 +90,12 @@ func (h *PaymentHandler) CreatePayment(c *gin.Context) {
 	notifyURL := fmt.Sprintf("%s/api/payment/bepusdt/notify", baseURL)
 	redirectURL := fmt.Sprintf("%s/orders/%d", baseURL, order.ID)
 
+	// 格式化amount，确保签名和请求中的格式一致
+	amountStr := strconv.FormatFloat(order.TotalAmount, 'f', 2, 64)
+
 	params := map[string]string{
 		"order_id":     strconv.Itoa(int(order.ID)),
-		"amount":       fmt.Sprint(order.TotalAmount),
+		"amount":       amountStr,
 		"notify_url":   notifyURL,
 		"redirect_url": redirectURL,
 	}
@@ -108,7 +111,7 @@ func (h *PaymentHandler) CreatePayment(c *gin.Context) {
 	reqBody := BepUsdtCreateRequest{
 		TradeType:   tradeType,
 		OrderID:     strconv.Itoa(int(order.ID)),
-		Amount:      order.TotalAmount,
+		Amount:      amountStr,
 		NotifyURL:   notifyURL,
 		RedirectURL: redirectURL,
 		Signature:   signature,