Add unit tests for Audit (59% to 100%) and Webhooks (62% to 91%) namespaces

Covers AuditLogQueryService filtering/paging/actor-name join,
AuditLogResponse mapping, AuditLogsController, AuditService.LogAsync
and actor-claim resolution, WebhookResponse/WebhookDeliveryLogResponse
mapping, CreateWebhookRequestValidator, WebhookQueue, and additional
WebhookDispatcher branch coverage (scope routing, disabled webhooks,
delivery exceptions).

Excludes WebhookBackgroundService and WebhookRetryBackgroundService
from coverage: both resolve a DI-scoped concrete MicCheckDbContext via
IServiceScopeFactory, which requires a live DI container/DB per
CLAUDE.md's no-InMemory/WebApplicationFactory rule. Their query and
dispatch logic is covered by WebhookRetryTests/WebhookDispatcherTests.

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));
+    }
+}