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.NotificationOutbox.Tests/NotificationOutboxActorPurgeTests.cs

@@ -8,6 +8,7 @@ using ScadaLink.Commons.Interfaces.Repositories;
 using ScadaLink.Commons.Types.Enums;
 using ScadaLink.NotificationOutbox.Delivery;
 using ScadaLink.NotificationOutbox.Messages;
+using ScadaLink.NotificationOutbox.Tests.TestSupport;
 
 namespace ScadaLink.NotificationOutbox.Tests;
 
@@ -46,6 +47,7 @@ public class NotificationOutboxActorPurgeTests : TestKit
                 DispatchInterval = TimeSpan.FromHours(1),
                 PurgeInterval = TimeSpan.FromHours(1),
             },
+            new NoOpCentralAuditWriter(),
             NullLogger<NotificationOutboxActor>.Instance)));
     }