Add unit tests for Environments, Audit, and Webhooks namespaces (#6)

## Summary
- Add missing unit test coverage for the Environments, Audit, and Webhooks namespaces (raises them from ~0-62% to 91-100%)
- Exclude WebhookBackgroundService/WebhookRetryBackgroundService from coverage (require live DI/DB, disallowed by CLAUDE.md's no-InMemory/WebApplicationFactory rule)

## Test plan
- [x] `dotnet test` full suite passes (646/646)
- [x] Coverage report confirms Environments ~99.5%, Audit 100%, Webhooks ~91%

Reviewed-on: #6

tests/api/MicCheck.Api.Tests.Unit/Common/Security/Authorization/AuthServiceTests.cs

@@ -216,4 +216,10 @@ public class AuthServiceTests
 
         Assert.That(response, Is.Null);
     }
+
+    [Test]
+    public void WhenLoggingOutWithATokenThatDoesNotExist_ThenNoExceptionIsThrown()
+    {
+        Assert.DoesNotThrowAsync(() => _service.LogoutAsync("unknown-token"));
+    }
 }