feat: add coupon checkout support

Tiku.IntegrationTests/Api/CommerceEndpointTests.cs

@@ -164,6 +164,87 @@ public sealed class CommerceEndpointTests
         Assert.Equal(firstPayment!.Id, secondPayment!.Id);
     }
 
+    [Fact]
+    public async Task Student_can_claim_and_check_coupon()
+    {
+        await using var factory = new ApiTestFactory(paymentProviderGateway: new FakePaymentGateway());
+        var seed = await SeedLoginUserAsync(factory);
+        await factory.SeedAsync(new Coupon
+        {
+            TenantId = seed.TenantId,
+            Code = "SAVE5",
+            PlanId = seed.PlanId,
+            DiscountType = DiscountType.Fixed,
+            DiscountValue = 5,
+            MaxUses = 10
+        });
+        using var client = factory.CreateClient();
+        await LoginAsync(client, seed);
+
+        var claimResponse = await client.PostAsJsonAsync(
+            "/api/commerce/coupons/claim",
+            new ClaimCommerceCouponDto { CouponCode = "SAVE5" });
+        var coupons = await (await client.GetAsync("/api/commerce/coupons"))
+            .Content
+            .ReadFromJsonAsync<CommerceCouponList>();
+        var check = await (await client.PostAsJsonAsync(
+                "/api/commerce/coupons/check",
+                new CheckCommerceCouponDto
+                {
+                    CouponCode = "SAVE5",
+                    PlanId = seed.PlanId,
+                    Quantity = 1
+                }))
+            .Content
+            .ReadFromJsonAsync<CommerceCouponCheckResult>();
+
+        Assert.Equal(HttpStatusCode.OK, claimResponse.StatusCode);
+        Assert.Contains(coupons!.Items, item => item.CouponCode == "SAVE5" && item.Status == "Claimed");
+        Assert.True(check!.Valid);
+        Assert.Equal(500, check.DiscountCents);
+        Assert.Equal(499, check.PayableAmountCents);
+    }
+
+    [Fact]
+    public async Task Coupon_can_make_order_zero_amount_and_grant_entitlement()
+    {
+        await using var factory = new ApiTestFactory(paymentProviderGateway: new FakePaymentGateway());
+        var seed = await SeedLoginUserAsync(factory);
+        await factory.SeedAsync(new Coupon
+        {
+            TenantId = seed.TenantId,
+            Code = "FREE",
+            PlanId = seed.PlanId,
+            DiscountType = DiscountType.Fixed,
+            DiscountValue = 9.99m,
+            MaxUses = 10
+        });
+        using var client = factory.CreateClient();
+        await LoginAsync(client, seed);
+
+        var response = await client.PostAsJsonAsync(
+            "/api/commerce/orders",
+            new CreateCommerceOrderDto
+            {
+                PlanId = seed.PlanId,
+                Quantity = 1,
+                CouponCode = "FREE"
+            });
+        var order = await response.Content.ReadFromJsonAsync<CommerceOrderItem>();
+        var entitlement = await (await client.GetAsync("/api/commerce/entitlements/current"))
+            .Content
+            .ReadFromJsonAsync<CurrentEntitlementItem>();
+        var coupons = await (await client.GetAsync("/api/commerce/coupons?status=used"))
+            .Content
+            .ReadFromJsonAsync<CommerceCouponList>();
+
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+        Assert.Equal(0, order!.AmountCents);
+        Assert.Equal("Paid", order.Status);
+        Assert.True(entitlement!.IsActive);
+        Assert.Contains(coupons!.Items, item => item.CouponCode == "FREE" && item.DiscountPreviewCents == 999);
+    }
+
     [Fact]
     public async Task Payment_notification_marks_order_paid_once()
     {