支持 .env 配置，并完善源头对接文档与测试订单

- 增加 .env.example 与 godotenv 加载，start.sh 自动读环境变量
- 重写发给源头的开放接口对接文档
- 订单页支持创建测试订单（可直接已支付并复制店铺订单号）

backend/internal/handler/order.go

@@ -45,30 +45,43 @@ func (h *OrderHandler) List(c *gin.Context) {
 }
 
 type createOrderReq struct {
-	SkinID    uint   `json:"skin_id" binding:"required"`
-	BuyerName string `json:"buyer_name"`
-	Remark    string `json:"remark"`
+	SkinID         uint   `json:"skin_id" binding:"required"`
+	BuyerName      string `json:"buyer_name"`
+	Remark         string `json:"remark"`
+	Status         string `json:"status"`          // 管理员可传 paid 直接可发货
+	DistributorID  *uint  `json:"distributor_id"` // 管理员可指定分销商
 }
 
 func (h *OrderHandler) Create(c *gin.Context) {
 	var req createOrderReq
 	if err := c.ShouldBindJSON(&req); err != nil {
-		response.BadRequest(c, "参数错误")
+		response.BadRequest(c, "参数错误：请选择商品")
 		return
 	}
 	distributorID := middleware.GetUserID(c)
-	// 管理员可指定分销商
+	status := ""
+	// 管理员可指定分销商、创建已支付测试单
 	if middleware.GetRole(c) == model.RoleAdmin {
+		if req.DistributorID != nil && *req.DistributorID > 0 {
+			distributorID = *req.DistributorID
+		}
 		if d := c.Query("distributor_id"); d != "" {
 			id, _ := strconv.ParseUint(d, 10, 64)
 			distributorID = uint(id)
 		}
+		if req.Status == model.OrderStatusPaid {
+			status = model.OrderStatusPaid
+		}
+	}
+	if req.BuyerName == "" {
+		req.BuyerName = "测试买家"
 	}
 	order, err := h.svc.Create(service.CreateOrderInput{
 		SkinID:        req.SkinID,
 		DistributorID: distributorID,
 		BuyerName:     req.BuyerName,
 		Remark:        req.Remark,
+		Status:        status,
 	})
 	if err != nil {
 		response.BadRequest(c, err.Error())