feat: add coupon support to orders and create user_coupons model

- Added CouponID field to Order model to track used coupons.
- Updated order query generation to include CouponID.
- Introduced UserCoupon model to manage user coupon associations.
- Implemented query methods for UserCoupon to facilitate CRUD operations.
- Updated query context and default query setup to include UserCoupon.

backend/app/http/v1/dto/order.go

@@ -1,9 +1,10 @@
 package dto
 
 type OrderCreateForm struct {
-	ContentID string `json:"content_id"`
-	Sku       string `json:"sku"`
-	Quantity  int    `json:"quantity"`
+	ContentID    string `json:"content_id"`
+	Sku          string `json:"sku"`
+	Quantity     int    `json:"quantity"`
+	UserCouponID string `json:"user_coupon_id"`
 }
 
 type OrderCreateResponse struct {