Fix integration test infrastructure

- Create CustomWebApplicationFactory to properly handle DI
- Fix IntegrationTestBase to use proper service scope pattern
- Update AuthIntegrationTests and EventsIntegrationTests to use CustomWebApplicationFactory
- Resolve EF Core provider conflict by using ConfigureWebHost override

Tests now run: 7 passed, 6 failed (auth/response format issues)

tests/integration/backend/AuthIntegrationTests.cs

@@ -1,14 +1,16 @@
 using System.Net;
 using System.Net.Http.Json;
 using FluentAssertions;
+using Microsoft.AspNetCore.Mvc.Testing;
 using RacePlannerApi.DTOs;
+using RacePlannerApi.Models;
 using Xunit;
 
 namespace backend.Tests.Integration;
 
 public class AuthIntegrationTests : IntegrationTestBase
 {
-    public AuthIntegrationTests(WebApplicationFactory<Program> factory) : base(factory) { }
+    public AuthIntegrationTests(CustomWebApplicationFactory factory) : base(factory) { }
 
     [Fact]
     public async Task Register_WithValidData_ReturnsSuccess()