撞车订单新增「接待中」状态，支持后台接待标记

已支付订单可标记为接待中，避免重复跟进；完成/取消覆盖接待中状态，前后台同步展示与筛选。

backend/internal/modules/mohong/handler.go

@@ -292,6 +292,21 @@ func (h *Handler) AdminGetOrder(c *gin.Context) {
 	response.OK(c, item)
 }
 
+func (h *Handler) AdminReceiveOrder(c *gin.Context) {
+	id, ok := parseID(c)
+	if !ok {
+		return
+	}
+	var req AdminReceiveOrderRequest
+	_ = c.ShouldBindJSON(&req)
+	item, err := h.service.AdminReceiveOrder(c.Request.Context(), id, req.Remark)
+	if err != nil {
+		writeError(c, err)
+		return
+	}
+	response.OK(c, item)
+}
+
 func (h *Handler) AdminCompleteOrder(c *gin.Context) {
 	id, ok := parseID(c)
 	if !ok {
@@ -357,6 +372,8 @@ func writeError(c *gin.Context, err error) {
 		response.Error(c, 409, "order_cannot_pay", "订单无法支付")
 	case errors.Is(err, ErrOrderCannotCancel):
 		response.Error(c, 409, "order_cannot_cancel", "订单无法取消")
+	case errors.Is(err, ErrOrderCannotReceive):
+		response.Error(c, 409, "order_cannot_receive", "订单无法标记为接待中")
 	case errors.Is(err, ErrOrderCannotComplete):
 		response.Error(c, 409, "order_cannot_complete", "订单无法完成")
 	case errors.Is(err, ErrInvalidRequest):