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/Environments/EnvironmentDocumentServiceTests.cs

@@ -0,0 +1,120 @@
+using MicCheck.Api.Data;
+using MicCheck.Api.Environments;
+using MicCheck.Api.Features;
+using MicCheck.Api.Organizations;
+using MicCheck.Api.Projects;
+using MicCheck.Api.Segments;
+using MicCheck.Api.Tests.Unit.TestSupport;
+using Moq;
+using NUnit.Framework;
+using AppEnvironment = MicCheck.Api.Environments.Environment;
+
+namespace MicCheck.Api.Tests.Unit.Environments;
+
+[TestFixture]
+public class EnvironmentDocumentServiceTests
+{
+    private Mock<IMicCheckDbContext> _db = null!;
+    private List<AppEnvironment> _environments = null!;
+    private List<Project> _projects = null!;
+    private List<Feature> _features = null!;
+    private List<FeatureState> _featureStates = null!;
+    private EnvironmentDocumentService _service = null!;
+    private const int ProjectId = 1;
+    private const int EnvironmentId = 1;
+
+    [SetUp]
+    public void SetUp()
+    {
+        _db = new Mock<IMicCheckDbContext>();
+
+        _environments = [new AppEnvironment { Id = EnvironmentId, Name = "Production", ApiKey = "env-key", ProjectId = ProjectId, CreatedAt = DateTimeOffset.UtcNow }];
+        var environmentsSet = MockDbSetFactory.Create(_environments);
+        environmentsSet.Setup(m => m.FindAsync(It.IsAny<object[]>(), It.IsAny<CancellationToken>()))
+            .ReturnsAsync((object[] keys, CancellationToken _) => _environments.FirstOrDefault(e => e.Id == (int)keys[0]));
+        _db.Setup(c => c.Environments).Returns(environmentsSet.Object);
+
+        _projects = [new Project { Id = ProjectId, Name = "Test Project", OrganizationId = 1, CreatedAt = DateTimeOffset.UtcNow }];
+        _db.SetupDbSet(c => c.Projects, _projects);
+        _db.SetupDbSet(c => c.Organizations, [new Organization { Id = 1, Name = "Org", CreatedAt = DateTimeOffset.UtcNow }]);
+
+        _features = [];
+        _db.SetupDbSet(c => c.Features, _features);
+        _featureStates = [];
+        _db.SetupDbSet(c => c.FeatureStates, _featureStates);
+
+        _service = new EnvironmentDocumentService(_db.Object);
+    }
+
+    [Test]
+    public async Task WhenTheEnvironmentDoesNotExist_ThenNullIsReturned()
+    {
+        var result = await _service.GetAsync(999);
+
+        Assert.That(result, Is.Null);
+    }
+
+    [Test]
+    public async Task WhenTheEnvironmentsProjectDoesNotExist_ThenNullIsReturned()
+    {
+        _projects.Clear();
+
+        var result = await _service.GetAsync(EnvironmentId);
+
+        Assert.That(result, Is.Null);
+    }
+
+    [Test]
+    public async Task WhenTheEnvironmentExists_ThenEnvironmentLevelFeatureStatesAreIncluded()
+    {
+        var feature = new Feature { Id = 1, Name = "flag_a", ProjectId = ProjectId, CreatedAt = DateTimeOffset.UtcNow };
+        _features.Add(feature);
+        _featureStates.Add(new FeatureState
+        {
+            FeatureId = feature.Id,
+            EnvironmentId = EnvironmentId,
+            Enabled = true,
+            Value = "on",
+            CreatedAt = DateTimeOffset.UtcNow,
+            UpdatedAt = DateTimeOffset.UtcNow
+        });
+
+        var result = await _service.GetAsync(EnvironmentId);
+
+        Assert.That(result, Is.Not.Null);
+        Assert.That(result!.Id, Is.EqualTo(EnvironmentId));
+        Assert.That(result.ApiKey, Is.EqualTo("env-key"));
+        Assert.That(result.FeatureStates, Has.Count.EqualTo(1));
+    }
+
+    [Test]
+    public async Task WhenTheEnvironmentExists_ThenIdentityAndSegmentScopedFeatureStatesAreExcluded()
+    {
+        var feature = new Feature { Id = 1, Name = "flag_a", ProjectId = ProjectId, CreatedAt = DateTimeOffset.UtcNow };
+        _features.Add(feature);
+        _featureStates.Add(new FeatureState
+        {
+            FeatureId = feature.Id,
+            EnvironmentId = EnvironmentId,
+            IdentityId = 5,
+            Enabled = true,
+            CreatedAt = DateTimeOffset.UtcNow,
+            UpdatedAt = DateTimeOffset.UtcNow
+        });
+
+        var result = await _service.GetAsync(EnvironmentId);
+
+        Assert.That(result!.FeatureStates, Is.Empty);
+    }
+
+    [Test]
+    public async Task WhenTheProjectHasSegments_ThenTheyAreIncludedInTheProjectResponse()
+    {
+        _projects[0].Segments.Add(new Segment { Id = 1, Name = "Beta Users", ProjectId = ProjectId, CreatedAt = DateTimeOffset.UtcNow });
+
+        var result = await _service.GetAsync(EnvironmentId);
+
+        Assert.That(result!.Project.Segments, Has.Count.EqualTo(1));
+        Assert.That(result.Project.Segments[0].Name, Is.EqualTo("Beta Users"));
+    }
+}