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/backend.Tests.Integration.csproj

@@ -0,0 +1,29 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" Version="6.0.4" />
+    <PackageReference Include="FluentAssertions" Version="8.9.0" />
+    <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="10.0.5" />
+    <PackageReference Include="Microsoft.AspNetCore.TestHost" Version="10.0.5" />
+    <PackageReference Include="Microsoft.EntityFrameworkCore.InMemory" Version="10.0.5" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
+    <PackageReference Include="xunit" Version="2.9.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.4" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\..\backend\RacePlannerApi.csproj" />
+  </ItemGroup>
+
+</Project>