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.

src/ScadaLink.NotificationOutbox/NotificationOutboxActor.cs

@@ -1,8 +1,10 @@
 using Akka.Actor;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Logging;
+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.Commons.Types.Notifications;
@@ -30,6 +32,7 @@ public class NotificationOutboxActor : ReceiveActor, IWithTimers
 
     private readonly IServiceProvider _serviceProvider;
     private readonly NotificationOutboxOptions _options;
+    private readonly ICentralAuditWriter _auditWriter;
     private readonly ILogger<NotificationOutboxActor> _logger;
 
     /// <summary>
@@ -45,11 +48,13 @@ public class NotificationOutboxActor : ReceiveActor, IWithTimers
     public NotificationOutboxActor(
         IServiceProvider serviceProvider,
         NotificationOutboxOptions options,
+        ICentralAuditWriter auditWriter,
         ILogger<NotificationOutboxActor> logger)
     {
-        _serviceProvider = serviceProvider;
-        _options = options;
-        _logger = logger;
+        _serviceProvider = serviceProvider ?? throw new ArgumentNullException(nameof(serviceProvider));
+        _options = options ?? throw new ArgumentNullException(nameof(options));
+        _auditWriter = auditWriter ?? throw new ArgumentNullException(nameof(auditWriter));
+        _logger = logger ?? throw new ArgumentNullException(nameof(logger));
 
         Receive<NotificationSubmit>(HandleSubmit);
         Receive<InternalMessages.IngestPersisted>(HandleIngestPersisted);