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/Identities/AdminIdentityResponseTests.cs

@@ -0,0 +1,34 @@
+using MicCheck.Api.Identities;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Identities;
+
+[TestFixture]
+public class AdminIdentityResponseTests
+{
+    [Test]
+    public void WhenMappingAnIdentityWithNoTraits_ThenTheResponseHasAnEmptyTraitsList()
+    {
+        var identity = new Identity { Id = 1, Identifier = "user-1", EnvironmentId = 5, CreatedAt = DateTimeOffset.UtcNow };
+
+        var response = AdminIdentityResponse.From(identity);
+
+        Assert.That(response.Id, Is.EqualTo(1));
+        Assert.That(response.Identifier, Is.EqualTo("user-1"));
+        Assert.That(response.EnvironmentId, Is.EqualTo(5));
+        Assert.That(response.Traits, Is.Empty);
+    }
+
+    [Test]
+    public void WhenMappingAnIdentityWithTraits_ThenEachTraitIsMapped()
+    {
+        var identity = new Identity { Id = 1, Identifier = "user-1", EnvironmentId = 1, CreatedAt = DateTimeOffset.UtcNow };
+        identity.Traits.Add(new IdentityTrait { Key = "plan", Value = "premium" });
+
+        var response = AdminIdentityResponse.From(identity);
+
+        Assert.That(response.Traits, Has.Count.EqualTo(1));
+        Assert.That(response.Traits[0].Key, Is.EqualTo("plan"));
+        Assert.That(response.Traits[0].Value, Is.EqualTo("premium"));
+    }
+}