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/Audit/AuditLogResponseTests.cs

@@ -0,0 +1,76 @@
+using MicCheck.Api.Audit;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Audit;
+
+[TestFixture]
+public class AuditLogResponseTests
+{
+    [Test]
+    public void WhenMappingWithoutAnActorUserName_ThenItDefaultsToNull()
+    {
+        var log = new AuditLog
+        {
+            Id = 1,
+            ResourceType = "Feature",
+            ResourceId = "42",
+            Action = "created",
+            OrganizationId = 1,
+            CreatedAt = DateTimeOffset.UtcNow
+        };
+
+        var response = AuditLogResponse.From(log);
+
+        Assert.That(response.ActorUserName, Is.Null);
+    }
+
+    [Test]
+    public void WhenMappingWithAnActorUserName_ThenItIsIncluded()
+    {
+        var log = new AuditLog
+        {
+            Id = 1,
+            ResourceType = "Feature",
+            ResourceId = "42",
+            Action = "created",
+            OrganizationId = 1,
+            ActorUserId = 7,
+            CreatedAt = DateTimeOffset.UtcNow
+        };
+
+        var response = AuditLogResponse.From(log, "Alice Smith");
+
+        Assert.That(response.ActorUserId, Is.EqualTo(7));
+        Assert.That(response.ActorUserName, Is.EqualTo("Alice Smith"));
+    }
+
+    [Test]
+    public void WhenMapping_ThenAllFieldsAreCopiedFromTheLog()
+    {
+        var createdAt = DateTimeOffset.UtcNow;
+        var log = new AuditLog
+        {
+            Id = 1,
+            ResourceType = "FeatureState",
+            ResourceId = "5",
+            Action = "updated",
+            Changes = """{"before":{},"after":{}}""",
+            OrganizationId = 10,
+            ProjectId = 20,
+            EnvironmentId = 30,
+            CreatedAt = createdAt
+        };
+
+        var response = AuditLogResponse.From(log);
+
+        Assert.That(response.Id, Is.EqualTo(1));
+        Assert.That(response.ResourceType, Is.EqualTo("FeatureState"));
+        Assert.That(response.ResourceId, Is.EqualTo("5"));
+        Assert.That(response.Action, Is.EqualTo("updated"));
+        Assert.That(response.Changes, Is.EqualTo("""{"before":{},"after":{}}"""));
+        Assert.That(response.OrganizationId, Is.EqualTo(10));
+        Assert.That(response.ProjectId, Is.EqualTo(20));
+        Assert.That(response.EnvironmentId, Is.EqualTo(30));
+        Assert.That(response.CreatedAt, Is.EqualTo(createdAt));
+    }
+}