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/Features/FeatureResponseTests.cs

@@ -0,0 +1,88 @@
+using MicCheck.Api.Features;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Features;
+
+[TestFixture]
+public class FeatureResponseTests
+{
+    [Test]
+    public void WhenMappingAFeatureWithNoTags_ThenTheResponseHasAnEmptyTagsList()
+    {
+        var feature = new Feature { Id = 1, Name = "dark_mode", ProjectId = 5, Type = FeatureType.Standard, CreatedAt = DateTimeOffset.UtcNow };
+
+        var response = FeatureResponse.From(feature);
+
+        Assert.That(response.Id, Is.EqualTo(1));
+        Assert.That(response.Name, Is.EqualTo("dark_mode"));
+        Assert.That(response.ProjectId, Is.EqualTo(5));
+        Assert.That(response.Tags, Is.Empty);
+    }
+
+    [Test]
+    public void WhenMappingAFeature_ThenTypeIsUpperInvariant()
+    {
+        var feature = new Feature { Id = 1, Name = "flag", ProjectId = 1, Type = FeatureType.MultiVariate, CreatedAt = DateTimeOffset.UtcNow };
+
+        var response = FeatureResponse.From(feature);
+
+        Assert.That(response.Type, Is.EqualTo("MULTIVARIATE"));
+    }
+
+    [Test]
+    public void WhenMappingAFeatureWithTags_ThenEachTagIsMapped()
+    {
+        var feature = new Feature { Id = 1, Name = "flag", ProjectId = 1, CreatedAt = DateTimeOffset.UtcNow };
+        feature.Tags.Add(new Tag { Id = 1, Label = "beta", Color = "#FF0000", ProjectId = 1 });
+
+        var response = FeatureResponse.From(feature);
+
+        Assert.That(response.Tags, Has.Count.EqualTo(1));
+        Assert.That(response.Tags[0].Label, Is.EqualTo("beta"));
+    }
+}
+
+[TestFixture]
+public class TagResponseTests
+{
+    [Test]
+    public void WhenMappingATag_ThenAllFieldsAreCopied()
+    {
+        var tag = new Tag { Id = 1, Label = "beta", Color = "#FF0000", ProjectId = 5 };
+
+        var response = TagResponse.From(tag);
+
+        Assert.That(response.Id, Is.EqualTo(1));
+        Assert.That(response.Label, Is.EqualTo("beta"));
+        Assert.That(response.Color, Is.EqualTo("#FF0000"));
+        Assert.That(response.ProjectId, Is.EqualTo(5));
+    }
+}
+
+[TestFixture]
+public class FeatureStateResponseTests
+{
+    [Test]
+    public void WhenMappingAFeatureState_ThenAllFieldsAreCopied()
+    {
+        var now = DateTimeOffset.UtcNow;
+        var state = new FeatureState
+        {
+            Id = 1,
+            FeatureId = 2,
+            EnvironmentId = 3,
+            Enabled = true,
+            Value = "on",
+            CreatedAt = now,
+            UpdatedAt = now
+        };
+
+        var response = FeatureStateResponse.From(state);
+
+        Assert.That(response.Id, Is.EqualTo(1));
+        Assert.That(response.FeatureId, Is.EqualTo(2));
+        Assert.That(response.EnvironmentId, Is.EqualTo(3));
+        Assert.That(response.Enabled, Is.True);
+        Assert.That(response.Value, Is.EqualTo("on"));
+    }
+}