CUB-53: add SignalR hub registration and configuration

backend/Hubs/AgentStatusHub.cs

@@ -0,0 +1,52 @@
+using Microsoft.AspNetCore.SignalR;
+
+namespace ControlCenter.Api.Hubs;
+
+/// <summary>
+/// SignalR hub for broadcasting agent status updates to connected clients.
+///
+/// <para>
+/// Clients connect to this hub at the <c>/hub</c> endpoint to receive
+/// real-time agent state changes. A background service subscribes to
+/// OpenClaw Gateway events and pushes them through this hub.
+/// </para>
+///
+/// <para>
+/// Architecture note: This hub bridges OpenClaw Gateway events to SignalR clients.
+/// The full typed client interface and extension methods will be added in a
+/// subsequent task (CUB-55).
+/// </para>
+/// </summary>
+public class AgentStatusHub : Hub
+{
+    private readonly ILogger<AgentStatusHub> _logger;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="AgentStatusHub"/> class.
+    /// </summary>
+    /// <param name="logger">Logger for diagnostic output.</param>
+    public AgentStatusHub(ILogger<AgentStatusHub> logger)
+    {
+        _logger = logger;
+    }
+
+    /// <summary>
+    /// Overrides <see cref="Hub.OnConnectedAsync"/> to log new connections.
+    /// </summary>
+    public override Task OnConnectedAsync()
+    {
+        _logger.LogDebug("Client connected: {ConnectionId}", Context.ConnectionId);
+        return base.OnConnectedAsync();
+    }
+
+    /// <summary>
+    /// Overrides <see cref="Hub.OnDisconnectedAsync"/> to log disconnections.
+    /// SignalR automatically removes disconnected connections from all groups.
+    /// </summary>
+    /// <param name="exception">Exception that caused the disconnection, if any.</param>
+    public override Task OnDisconnectedAsync(Exception? exception)
+    {
+        _logger.LogDebug("Client disconnected: {ConnectionId}", Context.ConnectionId);
+        return base.OnDisconnectedAsync(exception);
+    }
+}