fix(auditlog): populate the Actor column on outbound and central rows

Per the Audit Log Actor-column spec, Actor should carry the calling script
identity on outbound rows (ApiCall, DbWrite, NotifySend) and a system identity
on central-dispatch rows (NotifyDeliver). The original emission code hard-coded
Actor=null at all four sites, so only Inbound API rows (API key name) ever
filled it. Stamp the script identity and 'system' respectively.

src/ScadaLink.NotificationOutbox/NotificationOutboxActor.cs

@@ -30,6 +30,13 @@ public class NotificationOutboxActor : ReceiveActor, IWithTimers
     private const int FallbackMaxRetries = 10;
     private static readonly TimeSpan FallbackRetryDelay = TimeSpan.FromMinutes(1);
 
+    /// <summary>
+    /// Audit <c>Actor</c> stamped on central-dispatch (<c>NotifyDeliver</c>) rows.
+    /// The Actor-column spec assigns central-originated audit rows a system
+    /// identity — there is no per-call authenticated user at dispatch time.
+    /// </summary>
+    private const string SystemActor = "system";
+
     private readonly IServiceProvider _serviceProvider;
     private readonly NotificationOutboxOptions _options;
     private readonly ICentralAuditWriter _auditWriter;
@@ -500,9 +507,11 @@ public class NotificationOutboxActor : ReceiveActor, IWithTimers
             Channel = AuditChannel.Notification,
             Kind = AuditKind.NotifyDeliver,
             CorrelationId = correlationId,
-            // Central dispatch — no authenticated actor (the originating
-            // script's identity is captured on the upstream NotifySend row).
-            Actor = null,
+            // Central dispatch — a system identity per the Actor-column spec;
+            // there is no per-call authenticated user here. The originating
+            // script is still captured on SourceScript (and on the upstream
+            // NotifySend row).
+            Actor = SystemActor,
             SourceSiteId = notification.SourceSiteId,
             SourceInstanceId = notification.SourceInstanceId,
             SourceScript = notification.SourceScript,