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_test.go

@@ -0,0 +1,111 @@
+package services
+
+import (
+	"context"
+	"database/sql"
+	"testing"
+
+	"quyun/v2/app/commands/testx"
+	order_dto "quyun/v2/app/http/v1/dto"
+	"quyun/v2/database"
+	"quyun/v2/database/models"
+	"quyun/v2/pkg/consts"
+	"quyun/v2/providers/storage"
+
+	. "github.com/smartystreets/goconvey/convey"
+	"github.com/spf13/cast"
+	"github.com/stretchr/testify/suite"
+	"go.ipao.vip/atom/contracts"
+	"go.uber.org/dig"
+)
+
+type CouponTestSuiteInjectParams struct {
+	dig.In
+
+	DB       *sql.DB
+	Initials []contracts.Initial `group:"initials"`
+}
+
+type CouponTestSuite struct {
+	suite.Suite
+	CouponTestSuiteInjectParams
+}
+
+func Test_Coupon(t *testing.T) {
+	providers := testx.Default().With(Provide).With(storage.Provide)
+
+	testx.Serve(providers, t, func(p CouponTestSuiteInjectParams) {
+		suite.Run(t, &CouponTestSuite{CouponTestSuiteInjectParams: p})
+	})
+}
+
+func (s *CouponTestSuite) Test_CouponFlow() {
+	Convey("Coupon Flow", s.T(), func() {
+		ctx := s.T().Context()
+		database.Truncate(ctx, s.DB, models.TableNameCoupon, models.TableNameUserCoupon, models.TableNameOrder, models.TableNameUser, models.TableNameContent, models.TableNameContentPrice)
+
+		user := &models.User{Username: "coupon_user", Phone: "13800000001"}
+		models.UserQuery.WithContext(ctx).Create(user)
+
+		// 1. Create Coupon (Fixed 5.00 CNY, Min 10.00 CNY)
+		cp := &models.Coupon{
+			Title:          "Save 5",
+			Type:           "fix_amount",
+			Value:          500,
+			MinOrderAmount: 1000,
+		}
+		models.CouponQuery.WithContext(ctx).Create(cp)
+
+		// 2. Give to User
+		uc := &models.UserCoupon{
+			UserID:   user.ID,
+			CouponID: cp.ID,
+			Status:   "unused",
+		}
+		models.UserCouponQuery.WithContext(ctx).Create(uc)
+
+		Convey("should validate coupon successfully", func() {
+			discount, err := Coupon.Validate(ctx, user.ID, uc.ID, 1500)
+			So(err, ShouldBeNil)
+			So(discount, ShouldEqual, 500)
+		})
+
+		Convey("should fail if below min amount", func() {
+			_, err := Coupon.Validate(ctx, user.ID, uc.ID, 800)
+			So(err, ShouldNotBeNil)
+		})
+
+		Convey("should apply in Order.Create", func() {
+			// Setup Content
+			c := &models.Content{UserID: 99, Title: "Test", Status: consts.ContentStatusPublished}
+			models.ContentQuery.WithContext(ctx).Create(c)
+			models.ContentPriceQuery.WithContext(ctx).Create(&models.ContentPrice{
+				ContentID:   c.ID,
+				PriceAmount: 2000, // 20.00 CNY
+				Currency:    "CNY",
+			})
+
+			form := &order_dto.OrderCreateForm{
+				ContentID:    cast.ToString(c.ID),
+				UserCouponID: cast.ToString(uc.ID),
+			}
+			// Simulate Auth context for Order service
+			authCtx := context.WithValue(ctx, consts.CtxKeyUser, user.ID)
+			res, err := Order.Create(authCtx, form)
+			So(err, ShouldBeNil)
+
+			// Verify Order
+			oid := cast.ToInt64(res.OrderID)
+			o, _ := models.OrderQuery.WithContext(ctx).Where(models.OrderQuery.ID.Eq(oid)).First()
+			So(o.AmountOriginal, ShouldEqual, 2000)
+			So(o.AmountDiscount, ShouldEqual, 500)
+			So(o.AmountPaid, ShouldEqual, 1500)
+			So(o.CouponID, ShouldEqual, cp.ID)
+
+			// Verify Coupon Status
+			ucReload, _ := models.UserCouponQuery.WithContext(ctx).Where(models.UserCouponQuery.ID.Eq(uc.ID)).First()
+			So(ucReload.Status, ShouldEqual, "used")
+			So(ucReload.OrderID, ShouldEqual, oid)
+		})
+	})
+}