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.

tests/ScadaLink.NotificationOutbox.Tests/NotificationOutboxActorAttemptEmissionTests.cs

@@ -155,8 +155,8 @@ public class NotificationOutboxActorAttemptEmissionTests : TestKit
             Assert.Equal("site-alpha", evt.SourceSiteId);
             Assert.Equal("instance-42", evt.SourceInstanceId);
             Assert.Equal("AlarmScript", evt.SourceScript);
-            // Central dispatch: actor is null (no authenticated end-user).
-            Assert.Null(evt.Actor);
+            // Central dispatch: Actor is the system identity (no per-call user).
+            Assert.Equal("system", evt.Actor);
             // Successful attempt: no error message.
             Assert.Null(evt.ErrorMessage);
         });