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/appsettings.Production.json

@@ -0,0 +1,17 @@
+{
+  "Logging": {
+    "LogLevel": {
+      "Default": "Information",
+      "Microsoft.AspNetCore": "Warning"
+    }
+  },
+  "AllowedHosts": "*",
+  "Jwt": {
+    "Key": "${JWT_SECRET_KEY}",
+    "Issuer": "RacePlannerApi",
+    "Audience": "RacePlannerClient"
+  },
+  "ConnectionStrings": {
+    "DefaultConnection": "${DATABASE_URL}"
+  }
+}