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/backend.Tests.Integration.runtimeconfig.json

@@ -0,0 +1,20 @@
+{
+  "runtimeOptions": {
+    "tfm": "net10.0",
+    "frameworks": [
+      {
+        "name": "Microsoft.NETCore.App",
+        "version": "10.0.0"
+      },
+      {
+        "name": "Microsoft.AspNetCore.App",
+        "version": "10.0.0"
+      }
+    ],
+    "configProperties": {
+      "MSTest.EnableParentProcessQuery": true,
+      "System.Reflection.NullabilityInfoContext.IsSupported": true,
+      "System.Runtime.Serialization.EnableUnsafeBinaryFormatterSerialization": false
+    }
+  }
+}