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.AssemblyInfo.cs

@@ -0,0 +1,22 @@
+//------------------------------------------------------------------------------
+// <auto-generated>
+//     This code was generated by a tool.
+//
+//     Changes to this file may cause incorrect behavior and will be lost if
+//     the code is regenerated.
+// </auto-generated>
+//------------------------------------------------------------------------------
+
+using System;
+using System.Reflection;
+
+[assembly: System.Reflection.AssemblyCompanyAttribute("backend.Tests.Integration")]
+[assembly: System.Reflection.AssemblyConfigurationAttribute("Debug")]
+[assembly: System.Reflection.AssemblyFileVersionAttribute("1.0.0.0")]
+[assembly: System.Reflection.AssemblyInformationalVersionAttribute("1.0.0+d3ec22aa99708887598cc6e0dad1ae530bc4505c")]
+[assembly: System.Reflection.AssemblyProductAttribute("backend.Tests.Integration")]
+[assembly: System.Reflection.AssemblyTitleAttribute("backend.Tests.Integration")]
+[assembly: System.Reflection.AssemblyVersionAttribute("1.0.0.0")]
+
+// Von der MSBuild WriteCodeFragment-Klasse generiert.
+