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/services/coupon.go

@@ -0,0 +1,84 @@
+package services
+
+import (
+	"context"
+	"time"
+
+	"quyun/v2/app/errorx"
+	"quyun/v2/database/models"
+)
+
+// @provider
+type coupon struct{}
+
+// Validate checks if a coupon can be used for an order and returns the discount amount
+func (s *coupon) Validate(ctx context.Context, userID, userCouponID, amount int64) (int64, error) {
+	uc, err := models.UserCouponQuery.WithContext(ctx).Where(models.UserCouponQuery.ID.Eq(userCouponID)).First()
+	if err != nil {
+		return 0, errorx.ErrRecordNotFound.WithMsg("优惠券不存在")
+	}
+	if uc.UserID != userID {
+		return 0, errorx.ErrUnauthorized.WithMsg("无权使用该优惠券")
+	}
+	if uc.Status != "unused" {
+		return 0, errorx.ErrBusinessLogic.WithMsg("优惠券已使用或失效")
+	}
+
+	c, err := models.CouponQuery.WithContext(ctx).Where(models.CouponQuery.ID.Eq(uc.CouponID)).First()
+	if err != nil {
+		return 0, errorx.ErrRecordNotFound.WithMsg("优惠券信息缺失")
+	}
+
+	now := time.Now()
+	if !c.StartAt.IsZero() && now.Before(c.StartAt) {
+		return 0, errorx.ErrBusinessLogic.WithMsg("优惠券尚未生效")
+	}
+	if !c.EndAt.IsZero() && now.After(c.EndAt) {
+		return 0, errorx.ErrBusinessLogic.WithMsg("优惠券已过期")
+	}
+
+	if amount < c.MinOrderAmount {
+		return 0, errorx.ErrBusinessLogic.WithMsg("未达到优惠券使用门槛")
+	}
+
+	var discount int64
+	if c.Type == "fix_amount" {
+		discount = c.Value
+	} else if c.Type == "discount" {
+		discount = (amount * c.Value) / 100
+		if c.MaxDiscount > 0 && discount > c.MaxDiscount {
+			discount = c.MaxDiscount
+		}
+	}
+
+	// Discount cannot exceed order amount
+	if discount > amount {
+		discount = amount
+	}
+
+	return discount, nil
+}
+
+// MarkUsed marks a user coupon as used (intended to be called inside a transaction)
+func (s *coupon) MarkUsed(ctx context.Context, tx *models.Query, userCouponID, orderID int64) error {
+	now := time.Now()
+	// Update User Coupon
+	info, err := tx.UserCoupon.WithContext(ctx).Where(tx.UserCoupon.ID.Eq(userCouponID), tx.UserCoupon.Status.Eq("unused")).Updates(&models.UserCoupon{
+		Status:  "used",
+		OrderID: orderID,
+		UsedAt:  now,
+	})
+	if err != nil {
+		return err
+	}
+	if info.RowsAffected == 0 {
+		return errorx.ErrBusinessLogic.WithMsg("优惠券核销失败")
+	}
+
+	// Update Coupon used quantity (Optional, but good for stats)
+	// We need CouponID from uc
+	uc, _ := tx.UserCoupon.WithContext(ctx).Where(tx.UserCoupon.ID.Eq(userCouponID)).First()
+	_, _ = tx.Coupon.WithContext(ctx).Where(tx.Coupon.ID.Eq(uc.CouponID)).UpdateSimple(tx.Coupon.UsedQuantity.Add(1))
+
+	return nil
+}