优化回调推送：参考微信/支付宝机制，限制每商户单一回调地址

- 回调重试改为微信式固定退避序列（15s/15s/30s/3m/10m/20m/30m/30m/30m/1h/3h/3h/3h/6h/6h），共 16 次尝试
- 重试次数/推送超时/退避序列可通过环境变量配置（CALLBACK_MAX_ATTEMPTS/CALLBACK_PUSH_TIMEOUT_SECONDS/CALLBACK_RETRY_SCHEDULE）
- 每个商户仅允许 1 个 active 回调订阅，创建前校验，先停用后新建
- 前端回调页提示推送策略与单地址限制
- docker-compose 透传回调相关配置

backend/cmd/server/main.go

@@ -20,6 +20,18 @@ import (
 	"gorm.io/gorm"
 )
 
+// secondsToDurations 把秒数列表转成 time.Duration 列表（回调重试序列）。
+func secondsToDurations(seconds []int) []time.Duration {
+	if len(seconds) == 0 {
+		return nil
+	}
+	out := make([]time.Duration, 0, len(seconds))
+	for _, s := range seconds {
+		out = append(out, time.Duration(s)*time.Second)
+	}
+	return out
+}
+
 func main() {
 	cfg := config.Load()
 	time.Local = timeutil.Location()
@@ -54,7 +66,11 @@ func main() {
 	tenantSvc := service.NewTenantService(db)
 	authSvc := service.NewAuthService(db, jm, tenantSvc)
 	userSvc := service.NewUserService(db, tenantSvc)
-	callbackSvc := service.NewCallbackService(db, codec)
+	callbackSvc := service.NewCallbackService(db, codec, service.WithCallbackConfig(service.CallbackConfig{
+		MaxAttempts:   cfg.CallbackMaxAttempts,
+		PushTimeout:   time.Duration(cfg.CallbackPushTimeoutSeconds) * time.Second,
+		RetrySchedule: secondsToDurations(cfg.CallbackRetryScheduleSeconds),
+	}))
 	fulfillmentSvc := service.NewFulfillmentService(db, callbackSvc)
 	merchantSvc := service.NewMerchantService(db, codec, tenantSvc)
 	deliverySvc := service.NewDeliveryService(