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/migrations/20251230091905_create_coupons.sql

@@ -0,0 +1,46 @@
+-- +goose Up
+-- +goose StatementBegin
+CREATE TABLE "coupons" (
+    "id" BIGSERIAL PRIMARY KEY,
+    "tenant_id" BIGINT NOT NULL DEFAULT 0, -- 租户ID
+    "title" VARCHAR(255) NOT NULL, -- 优惠券标题
+    "description" TEXT, -- 优惠券描述
+    "type" VARCHAR(32) NOT NULL, -- 优惠券类型: fix_amount(固定金额), discount(折扣比率)
+    "value" BIGINT NOT NULL, -- 优惠券面值 (固定金额为分，折扣比率为0-100)
+    "min_order_amount" BIGINT NOT NULL DEFAULT 0, -- 最低订单金额门槛 (分)
+    "max_discount" BIGINT, -- 最高抵扣金额 (分，针对折扣券)
+    "total_quantity" INTEGER NOT NULL DEFAULT 0, -- 发行总量
+    "used_quantity" INTEGER NOT NULL DEFAULT 0, -- 已使用数量
+    "start_at" TIMESTAMPTZ, -- 开始生效时间
+    "end_at" TIMESTAMPTZ, -- 过期失效时间
+    "created_at" TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    "updated_at" TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX "idx_coupons_tenant_id" ON "coupons" ("tenant_id");
+
+CREATE TABLE "user_coupons" (
+    "id" BIGSERIAL PRIMARY KEY,
+    "user_id" BIGINT NOT NULL, -- 用户ID
+    "coupon_id" BIGINT NOT NULL, -- 优惠券ID
+    "order_id" BIGINT, -- 使用该优惠券的订单ID
+    "status" VARCHAR(32) NOT NULL DEFAULT 'unused', -- 状态: unused(未使用), used(已使用), expired(已过期)
+    "used_at" TIMESTAMPTZ, -- 使用时间
+    "created_at" TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX "idx_user_coupons_user_id" ON "user_coupons" ("user_id");
+CREATE INDEX "idx_user_coupons_coupon_id" ON "user_coupons" ("coupon_id");
+
+-- Add CouponID to Orders table
+ALTER TABLE "orders" ADD COLUMN "coupon_id" BIGINT DEFAULT 0; -- 关联优惠券ID (0表示未使用)
+COMMENT ON COLUMN "orders"."coupon_id" IS '使用的优惠券ID (0表示未使用)';
+
+-- +goose StatementEnd
+
+-- +goose Down
+-- +goose StatementBegin
+ALTER TABLE "orders" DROP COLUMN IF EXISTS "coupon_id";
+DROP TABLE IF EXISTS "user_coupons";
+DROP TABLE IF EXISTS "coupons";
+-- +goose StatementEnd