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/EventsIntegrationTests.cs

@@ -1,6 +1,7 @@
 using System.Net;
 using System.Net.Http.Json;
 using FluentAssertions;
+using Microsoft.AspNetCore.Mvc.Testing;
 using RacePlannerApi.DTOs;
 using RacePlannerApi.Models;
 using Xunit;
@@ -9,7 +10,7 @@ namespace backend.Tests.Integration;
 
 public class EventsIntegrationTests : IntegrationTestBase
 {
-    public EventsIntegrationTests(WebApplicationFactory<Program> factory) : base(factory) { }
+    public EventsIntegrationTests(CustomWebApplicationFactory factory) : base(factory) { }
 
     private async Task<string> GetOrganizerTokenAsync()
     {