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/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()