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.Commons/Messages/DebugView/DebugStreamEvent.cs

@@ -0,0 +1,8 @@
+namespace ScadaLink.Commons.Messages.DebugView;
+
+/// <summary>
+/// Wraps a debug stream event (AttributeValueChanged or AlarmStateChanged) with
+/// the correlationId for routing back to the correct DebugStreamBridgeActor on central.
+/// Sent from InstanceActor → SiteCommunicationActor → ClusterClient → CentralCommunicationActor.
+/// </summary>
+public record DebugStreamEvent(string CorrelationId, object Event);