feat: finish back-end work (untested)

api/dto/item_order_set_quantity_request.go

@@ -0,0 +1,7 @@
+package dto
+
+type ItemOrderSetQuantityRequest struct {
+	OrderId  int `json:"order_id"`
+	ItemId   int `json:"item_id"`
+	Quantity int `json:"quantity"`
+}