data 透传闭环：发货数据回显、回调带回，并限制 2KB

- 「查询发货数据」响应新增 data（下单透传原样回显，供自建发货页预填/展示）
- 回调 payload 带回 data（下单传 → 回调收，商户对接免回查）
- 创建订单 data 限制最长 2048 字节
- 文档更新：data 用途（自建页预填/回调/上游查单约定字段）、发货数据与回调示例
- 单测覆盖 data 回显

backend/internal/service/delivery_test.go

@@ -221,6 +221,10 @@ func TestDeliveryMerchantApiBindAndSubmit(t *testing.T) {
 		APIClientID:   1,
 		ClientOrderNo: "delivery-api-001",
 		SKU:           "suit_alan_walker",
+		RequestData: map[string]interface{}{
+			"game_account": "4808146277",
+			"game_channel": "ios-wechat",
+		},
 	})
 	if err != nil {
 		t.Fatalf("create order: %v", err)
@@ -234,6 +238,9 @@ func TestDeliveryMerchantApiBindAndSubmit(t *testing.T) {
 	if !info.CanShip || info.Product == nil || info.Product.Image == "" {
 		t.Fatalf("merchant order should include delivery data: %+v", info)
 	}
+	if info.Data["game_account"] != "4808146277" || info.Data["game_channel"] != "ios-wechat" {
+		t.Fatalf("delivery order should echo request data: %+v", info.Data)
+	}
 
 	bind, err := deliverySvc.BindForMerchant(merchantID, created.Order.OrderNo, "4808146277")
 	if err != nil {