Add integration test infrastructure

- Create backend integration test project with xUnit
- Add required packages: TestHost, Mvc.Testing, EF InMemory, FluentAssertions
- Add project reference to backend API
- Create IntegrationTestBase class with WebApplicationFactory setup
- Install Supertest for HTTP integration testing

tests/integration/backend/obj/Debug/net10.0/backend.Tests.Integration.GlobalUsings.g.cs

@@ -0,0 +1,9 @@
+// <auto-generated/>
+global using System;
+global using System.Collections.Generic;
+global using System.IO;
+global using System.Linq;
+global using System.Net.Http;
+global using System.Threading;
+global using System.Threading.Tasks;
+global using Xunit;