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/bin/Debug/net10.0/MvcTestingAppManifest.json

@@ -0,0 +1,3 @@
+{
+  "RacePlannerApi, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null": "\/Users\/mastermito\/Dev\/raceplanner\/backend"
+}