fix: BepUsdt签名改用fmt.Sprint格式化amount(与官方一致)，添加调试日志

backend/internal/api/handlers/payment.go

@@ -78,9 +78,11 @@ func (h *PaymentHandler) CreatePayment(c *gin.Context) {
 	redirectURL := fmt.Sprintf("%s/orders/%d", baseURL, order.ID)
 	timestamp := time.Now().Unix()
 
+	amountStr := fmt.Sprint(order.TotalAmount)
+
 	params := map[string]string{
 		"order_id":     strconv.Itoa(int(order.ID)),
-		"amount":       strconv.FormatFloat(order.TotalAmount, 'f', -1, 64),
+		"amount":       amountStr,
 		"notify_url":   notifyURL,
 		"redirect_url": redirectURL,
 		"timestamp":    strconv.FormatInt(timestamp, 10),
@@ -91,6 +93,9 @@ func (h *PaymentHandler) CreatePayment(c *gin.Context) {
 
 	signature := signBepUsdt(params, apiToken)
 
+	fmt.Printf("[BepUsdt] signature params: %v\n", params)
+	fmt.Printf("[BepUsdt] signature: %s, token: %s\n", signature, apiToken)
+
 	reqBody := BepUsdtCreateRequest{
 		TradeType:   tradeType,
 		OrderID:     strconv.Itoa(int(order.ID)),
@@ -102,6 +107,7 @@ func (h *PaymentHandler) CreatePayment(c *gin.Context) {
 	}
 
 	body, _ := json.Marshal(reqBody)
+	fmt.Printf("[BepUsdt] request body: %s\n", string(body))
 	resp, err := http.Post(apiURL+"/api/v1/order/create-transaction", "application/json", strings.NewReader(string(body)))
 	if err != nil {
 		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to connect payment gateway"})
@@ -111,6 +117,8 @@ func (h *PaymentHandler) CreatePayment(c *gin.Context) {
 
 	respBody, _ := io.ReadAll(resp.Body)
 
+	fmt.Printf("[BepUsdt] response: %s\n", string(respBody))
+
 	var result map[string]interface{}
 	json.Unmarshal(respBody, &result)