feat(Backend): update API gateway and websocket hubs

FinlyticBackend/Hubs/SystemHealthHub.cs

@@ -0,0 +1,31 @@
+using System;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.SignalR;
+using Microsoft.Extensions.Logging;
+
+namespace FinlyticBackend.Hubs;
+
+/// <summary>
+/// SignalR Hub broadcasting real-time system diagnostics & microservice health updates to connected clients.
+/// </summary>
+public class SystemHealthHub : Hub
+{
+    private readonly ILogger<SystemHealthHub> _logger;
+
+    public SystemHealthHub(ILogger<SystemHealthHub> logger)
+    {
+        _logger = logger;
+    }
+
+    public override async Task OnConnectedAsync()
+    {
+        _logger.LogInformation("[SystemHealthHub] Client connected: ConnectionId={ConnectionId}", Context.ConnectionId);
+        await base.OnConnectedAsync();
+    }
+
+    public override async Task OnDisconnectedAsync(Exception? exception)
+    {
+        _logger.LogInformation("[SystemHealthHub] Client disconnected: ConnectionId={ConnectionId}", Context.ConnectionId);
+        await base.OnDisconnectedAsync(exception);
+    }
+}