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/EnvironmentDocumentControllerTests.cs

@@ -0,0 +1,69 @@
+using System.Security.Claims;
+using MicCheck.Api.Data;
+using MicCheck.Api.Environments;
+using MicCheck.Api.Organizations;
+using MicCheck.Api.Projects;
+using MicCheck.Api.Tests.Unit.TestSupport;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc;
+using Moq;
+using NUnit.Framework;
+using AppEnvironment = MicCheck.Api.Environments.Environment;
+
+namespace MicCheck.Api.Tests.Unit.Environments;
+
+[TestFixture]
+public class EnvironmentDocumentControllerTests
+{
+    private Mock<IMicCheckDbContext> _db = null!;
+    private List<AppEnvironment> _environments = null!;
+    private EnvironmentDocumentController _controller = null!;
+    private const int EnvironmentId = 1;
+    private const int ProjectId = 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);
+
+        _db.SetupDbSet(c => c.Projects, [new Project { Id = ProjectId, Name = "Test Project", OrganizationId = 1, CreatedAt = DateTimeOffset.UtcNow }]);
+        _db.SetupDbSet(c => c.Organizations, [new Organization { Id = 1, Name = "Org", CreatedAt = DateTimeOffset.UtcNow }]);
+        _db.SetupDbSet(c => c.Features, []);
+        _db.SetupDbSet(c => c.FeatureStates, []);
+
+        _controller = new EnvironmentDocumentController(new EnvironmentDocumentService(_db.Object));
+
+        var identity = new ClaimsIdentity([new Claim("EnvironmentId", EnvironmentId.ToString())], "TestAuth");
+        _controller.ControllerContext = new ControllerContext
+        {
+            HttpContext = new DefaultHttpContext { User = new ClaimsPrincipal(identity) }
+        };
+    }
+
+    [Test]
+    public async Task WhenTheEnvironmentDocumentExists_ThenItIsReturned()
+    {
+        var result = await _controller.Get(CancellationToken.None);
+
+        var ok = result.Result as OkObjectResult;
+        Assert.That(ok, Is.Not.Null);
+        var document = ok!.Value as EnvironmentDocumentResponse;
+        Assert.That(document!.Id, Is.EqualTo(EnvironmentId));
+    }
+
+    [Test]
+    public async Task WhenTheEnvironmentDocumentDoesNotExist_ThenNotFoundIsReturned()
+    {
+        _environments.Clear();
+
+        var result = await _controller.Get(CancellationToken.None);
+
+        Assert.That(result.Result, Is.InstanceOf<NotFoundResult>());
+    }
+}