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/Projects/ProjectResponseTests.cs

@@ -0,0 +1,45 @@
+using MicCheck.Api.Common.Security.Authorization;
+using MicCheck.Api.Projects;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Projects;
+
+[TestFixture]
+public class ProjectResponseTests
+{
+    [Test]
+    public void WhenMappingAProject_ThenAllFieldsAreCopied()
+    {
+        var project = new Project { Id = 1, Name = "My Project", OrganizationId = 5, HideDisabledFlags = true, CreatedAt = DateTimeOffset.UtcNow };
+
+        var response = ProjectResponse.From(project);
+
+        Assert.That(response.Id, Is.EqualTo(1));
+        Assert.That(response.Name, Is.EqualTo("My Project"));
+        Assert.That(response.OrganizationId, Is.EqualTo(5));
+        Assert.That(response.HideDisabledFlags, Is.True);
+    }
+}
+
+[TestFixture]
+public class UserPermissionResponseTests
+{
+    [Test]
+    public void WhenMappingAUserProjectPermission_ThenPermissionsAreMappedToStrings()
+    {
+        var permission = new UserProjectPermission
+        {
+            UserId = 1,
+            ProjectId = 2,
+            IsAdmin = true,
+            Permissions = [ProjectPermission.ViewProject, ProjectPermission.EditFeature]
+        };
+
+        var response = UserPermissionResponse.From(permission);
+
+        Assert.That(response.UserId, Is.EqualTo(1));
+        Assert.That(response.ProjectId, Is.EqualTo(2));
+        Assert.That(response.IsAdmin, Is.True);
+        Assert.That(response.Permissions, Is.EqualTo(new[] { "ViewProject", "EditFeature" }));
+    }
+}