feat: replace debug view polling with real-time SignalR streaming

The debug view polled every 2s by re-subscribing for full snapshots. Now a
persistent DebugStreamBridgeActor on central subscribes once and receives
incremental Akka stream events from the site, forwarding them to the Blazor
component via callbacks and to the CLI via a new SignalR hub at
/hubs/debug-stream. Adds `debug stream` CLI command with auto-reconnect.

src/ScadaLink.Host/Program.cs

@@ -138,6 +138,7 @@ try
         app.MapCentralUI<ScadaLink.Host.Components.App>();
         app.MapInboundAPI();
         app.MapManagementAPI();
+        app.MapHub<ScadaLink.ManagementService.DebugStreamHub>("/hubs/debug-stream");
 
         // Compile and register all Inbound API method scripts at startup
         using (var scope = app.Services.CreateScope())