fix(site-event-log): record script errors and route queries to the active node

Script execution failures were only written to Serilog, never to the
site event log — SiteRuntime did not reference the SiteEventLogging
project. ScriptExecutionActor now resolves ISiteEventLogger and emits a
'script'/'Error' event on timeout and exception.

The event-log query handler was a per-node actor bound to that node's
local SQLite. A ClusterClient query could land on the standby (which
records no events) and return nothing. The handler is now a cluster
singleton with a proxy, so queries always reach the active node.

src/ScadaLink.Host/Actors/AkkaHostedService.cs

@@ -306,14 +306,29 @@ akka {{
         // Register local handlers with SiteCommunicationActor
         siteCommActor.Tell(new RegisterLocalHandler(LocalHandlerType.Artifacts, dmProxy));
 
-        // Event log handler — bridges Akka to IEventLogQueryService
+        // Event log handler — cluster singleton so queries always reach the
+        // active node. The event log is node-local SQLite and is not
+        // replicated; only the active node records events. A per-node handler
+        // would let a ClusterClient query land on the standby and find nothing.
         var eventLogQueryService = _serviceProvider.GetService<SiteEventLogging.IEventLogQueryService>();
         if (eventLogQueryService != null)
         {
-            var eventLogHandler = _actorSystem.ActorOf(
-                Props.Create(() => new SiteEventLogging.EventLogHandlerActor(eventLogQueryService)),
-                "event-log-handler");
-            siteCommActor.Tell(new RegisterLocalHandler(LocalHandlerType.EventLog, eventLogHandler));
+            var eventLogSingletonProps = ClusterSingletonManager.Props(
+                singletonProps: Props.Create(() => new SiteEventLogging.EventLogHandlerActor(eventLogQueryService)),
+                terminationMessage: PoisonPill.Instance,
+                settings: ClusterSingletonManagerSettings.Create(_actorSystem)
+                    .WithRole(siteRole)
+                    .WithSingletonName("event-log-handler"));
+            _actorSystem.ActorOf(eventLogSingletonProps, "event-log-handler-singleton");
+
+            var eventLogProxyProps = ClusterSingletonProxy.Props(
+                singletonManagerPath: "/user/event-log-handler-singleton",
+                settings: ClusterSingletonProxySettings.Create(_actorSystem)
+                    .WithRole(siteRole)
+                    .WithSingletonName("event-log-handler"));
+            var eventLogProxy = _actorSystem.ActorOf(eventLogProxyProps, "event-log-handler-proxy");
+
+            siteCommActor.Tell(new RegisterLocalHandler(LocalHandlerType.EventLog, eventLogProxy));
         }
 
         // Parked message handler — bridges Akka to StoreAndForwardService