fix: route debug stream events through ClusterClient site→central path

ClusterClient Sender refs are temporary proxies — valid for immediate reply
but not durable for future Tells. Events now flow as DebugStreamEvent through
SiteCommunicationActor → ClusterClient → CentralCommunicationActor → bridge
actor (same pattern as health reports). Also fix DebugStreamHub to use
IHubContext for long-lived callbacks instead of transient hub instance.

src/ScadaLink.Communication/Actors/CentralCommunicationActor.cs

@@ -85,6 +85,9 @@ public class CentralCommunicationActor : ReceiveActor
 
         // Route enveloped messages to sites
         Receive<SiteEnvelope>(HandleSiteEnvelope);
+
+        // Route debug stream events from sites to the correct bridge actor
+        Receive<Commons.Messages.DebugView.DebugStreamEvent>(HandleDebugStreamEvent);
     }
 
     private void HandleHeartbeat(HeartbeatMessage heartbeat)
@@ -93,6 +96,18 @@ public class CentralCommunicationActor : ReceiveActor
         Context.Parent.Tell(heartbeat);
     }
 
+    private void HandleDebugStreamEvent(Commons.Messages.DebugView.DebugStreamEvent msg)
+    {
+        if (_debugSubscriptions.TryGetValue(msg.CorrelationId, out var entry))
+        {
+            entry.Subscriber.Tell(msg.Event);
+        }
+        else
+        {
+            _log.Debug("No debug subscription found for correlationId {0}, dropping event", msg.CorrelationId);
+        }
+    }
+
     private void HandleSiteHealthReport(SiteHealthReport report)
     {
         var aggregator = _serviceProvider.GetService<ICentralHealthAggregator>();

src/ScadaLink.Communication/Actors/SiteCommunicationActor.cs

@@ -145,6 +145,13 @@ public class SiteCommunicationActor : ReceiveActor, IWithTimers
             _centralClient?.Tell(
                 new ClusterClient.Send("/user/central-communication", msg), Self);
         });
+
+        // Internal: forward debug stream events to central (site→central streaming)
+        Receive<DebugStreamEvent>(msg =>
+        {
+            _centralClient?.Tell(
+                new ClusterClient.Send("/user/central-communication", msg), Self);
+        });
     }
 
     protected override void PreStart()