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/Webhooks/WebhookQueueTests.cs

@@ -0,0 +1,48 @@
+using MicCheck.Api.Webhooks;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Webhooks;
+
+[TestFixture]
+public class WebhookQueueTests
+{
+    [Test]
+    public async Task WhenAnEventIsEnqueued_ThenItCanBeReadBack()
+    {
+        var queue = new WebhookQueue();
+        var webhookEvent = new WebhookEvent
+        {
+            EventType = WebhookEventTypes.FlagUpdated,
+            OrganizationId = 1,
+            Data = new { }
+        };
+
+        await queue.EnqueueAsync(webhookEvent);
+
+        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(1));
+        await using var enumerator = queue.ReadAllAsync(cts.Token).GetAsyncEnumerator(cts.Token);
+        await enumerator.MoveNextAsync();
+
+        Assert.That(enumerator.Current, Is.SameAs(webhookEvent));
+    }
+
+    [Test]
+    public async Task WhenMultipleEventsAreEnqueued_ThenTheyAreReadInOrder()
+    {
+        var queue = new WebhookQueue();
+        var first = new WebhookEvent { EventType = WebhookEventTypes.FlagUpdated, OrganizationId = 1, Data = new { } };
+        var second = new WebhookEvent { EventType = WebhookEventTypes.FlagDeleted, OrganizationId = 1, Data = new { } };
+
+        await queue.EnqueueAsync(first);
+        await queue.EnqueueAsync(second);
+
+        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(1));
+        await using var enumerator = queue.ReadAllAsync(cts.Token).GetAsyncEnumerator(cts.Token);
+
+        await enumerator.MoveNextAsync();
+        Assert.That(enumerator.Current, Is.SameAs(first));
+
+        await enumerator.MoveNextAsync();
+        Assert.That(enumerator.Current, Is.SameAs(second));
+    }
+}