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/database/models/user_coupons.gen.go

@@ -0,0 +1,59 @@
+// Code generated by go.ipao.vip/gen. DO NOT EDIT.
+// Code generated by go.ipao.vip/gen. DO NOT EDIT.
+// Code generated by go.ipao.vip/gen. DO NOT EDIT.
+
+package models
+
+import (
+	"context"
+	"time"
+
+	"go.ipao.vip/gen"
+)
+
+const TableNameUserCoupon = "user_coupons"
+
+// UserCoupon mapped from table <user_coupons>
+type UserCoupon struct {
+	ID        int64     `gorm:"column:id;type:bigint;primaryKey;autoIncrement:true" json:"id"`
+	UserID    int64     `gorm:"column:user_id;type:bigint;not null" json:"user_id"`
+	CouponID  int64     `gorm:"column:coupon_id;type:bigint;not null" json:"coupon_id"`
+	OrderID   int64     `gorm:"column:order_id;type:bigint" json:"order_id"`
+	Status    string    `gorm:"column:status;type:character varying(32);not null;default:unused" json:"status"`
+	UsedAt    time.Time `gorm:"column:used_at;type:timestamp with time zone" json:"used_at"`
+	CreatedAt time.Time `gorm:"column:created_at;type:timestamp with time zone;not null;default:now()" json:"created_at"`
+}
+
+// Quick operations without importing query package
+// Update applies changed fields to the database using the default DB.
+func (m *UserCoupon) Update(ctx context.Context) (gen.ResultInfo, error) {
+	return Q.UserCoupon.WithContext(ctx).Updates(m)
+}
+
+// Save upserts the model using the default DB.
+func (m *UserCoupon) Save(ctx context.Context) error { return Q.UserCoupon.WithContext(ctx).Save(m) }
+
+// Create inserts the model using the default DB.
+func (m *UserCoupon) Create(ctx context.Context) error {
+	return Q.UserCoupon.WithContext(ctx).Create(m)
+}
+
+// Delete removes the row represented by the model using the default DB.
+func (m *UserCoupon) Delete(ctx context.Context) (gen.ResultInfo, error) {
+	return Q.UserCoupon.WithContext(ctx).Delete(m)
+}
+
+// ForceDelete permanently deletes the row (ignores soft delete) using the default DB.
+func (m *UserCoupon) ForceDelete(ctx context.Context) (gen.ResultInfo, error) {
+	return Q.UserCoupon.WithContext(ctx).Unscoped().Delete(m)
+}
+
+// Reload reloads the model from database by its primary key and overwrites current fields.
+func (m *UserCoupon) Reload(ctx context.Context) error {
+	fresh, err := Q.UserCoupon.WithContext(ctx).GetByID(m.ID)
+	if err != nil {
+		return err
+	}
+	*m = *fresh
+	return nil
+}