Extract IAuditService, drop virtual mock seam from AuditService

Consumers now depend on IAuditService instead of the concrete class,
so tests mock the interface (Mock<IAuditService>()) rather than
subclassing AuditService via virtual methods.

tests/api/MicCheck.Api.Tests.Unit/Segments/SegmentsControllerTests.cs

@@ -41,7 +41,7 @@ public class SegmentsControllerTests
         _db.SetupDbSetWithGeneratedIds(c => c.SegmentConditions, _segmentConditions);
 
         var webhookQueue = new MicCheck.Api.Webhooks.WebhookQueue();
-        var auditService = new Mock<AuditService>(_db.Object, null!, webhookQueue);
+        var auditService = new Mock<IAuditService>();
         auditService.Setup(a => a.LogAsync(
             It.IsAny<string>(), It.IsAny<string>(), It.IsAny<string>(),
             It.IsAny<int>(), It.IsAny<int?>(), It.IsAny<int?>(),