feat(notif): NotificationOutboxActor + CentralAuditWriter wired (#23 M4)

M4 Bundle B (B1) — add the central-only ICentralAuditWriter implementation
and inject it into NotificationOutboxActor so subsequent tasks (B2/B3) can
route attempt + terminal lifecycle events through the direct-write audit path.

- CentralAuditWriter: thin wrapper around IAuditLogRepository.InsertIfNotExistsAsync;
  scope-per-call (matches AuditLogIngestActor / NotificationOutboxActor pattern);
  stamps IngestedAtUtc; swallows all internal failures (alog.md §13).
- Registered as a singleton in AddAuditLog.
- NotificationOutboxActor ctor takes ICentralAuditWriter (validated non-null).
- Host wiring resolves the writer once from the root provider and passes it
  into the singleton's Props.Create call.
- Existing TestKit fixtures updated with a NoOpCentralAuditWriter helper so
  tests that don't exercise audit emission still compile and pass.

tests/ScadaLink.IntegrationTests/NotificationOutboxFlowTests.cs

@@ -8,8 +8,10 @@ using Microsoft.EntityFrameworkCore.Storage.ValueConversion;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Logging.Abstractions;
 using NSubstitute;
+using ScadaLink.Commons.Entities.Audit;
 using ScadaLink.Commons.Entities.Notifications;
 using ScadaLink.Commons.Interfaces.Repositories;
+using ScadaLink.Commons.Interfaces.Services;
 using ScadaLink.Commons.Messages.Notification;
 using ScadaLink.Commons.Types.Enums;
 using ScadaLink.ConfigurationDatabase;
@@ -172,7 +174,20 @@ public class NotificationOutboxFlowTests : TestKit
         };
 
         return Sys.ActorOf(Props.Create(() => new NotificationOutboxActor(
-            services, options, NullLogger<NotificationOutboxActor>.Instance)));
+            services,
+            options,
+            (ICentralAuditWriter)new NoOpCentralAuditWriter(),
+            NullLogger<NotificationOutboxActor>.Instance)));
+    }
+
+    /// <summary>
+    /// Test-only no-op <see cref="ICentralAuditWriter"/>. The integration tests
+    /// in this file pre-date M4 Bundle B's audit-writer injection; they do not
+    /// assert on emission, just need a non-null collaborator.
+    /// </summary>
+    private sealed class NoOpCentralAuditWriter : ICentralAuditWriter
+    {
+        public Task WriteAsync(AuditEvent evt, CancellationToken ct = default) => Task.CompletedTask;
     }
 
     private static NotificationSubmit MakeSubmit(string notificationId)