feat: implement Phase 4 — SignalR real-time updates

Backend:
- RmmHub with typed IRmmHubClient interface (AgentMetricsUpdated, AgentStatusChanged, CommandResultUpdated)
- JoinAgentGroup / LeaveAgentGroup for per-agent subscriptions
- AgentGrpcService now pushes to SignalR after every Heartbeat and CommandResult
- Program.cs maps /hubs/rmm

Frontend:
- useSignalR hook with exponential backoff reconnect (0s/2s/10s/30s)
- useGlobalSignalR: invalidates agents query on AgentStatusChanged
- useAgentSignalR: joins agent group, invalidates metrics/tasks on updates
- DashboardPage: live agent status updates via SignalR
- AgentDetailPage: live metrics/command results + connection status indicator

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Backend/src/NexusRMM.Api/Hubs/RmmHub.cs

@@ -0,0 +1,38 @@
+using Microsoft.AspNetCore.SignalR;
+
+namespace NexusRMM.Api.Hubs;
+
+public class RmmHub : Hub<IRmmHubClient>
+{
+    private readonly ILogger<RmmHub> _logger;
+
+    public RmmHub(ILogger<RmmHub> logger)
+    {
+        _logger = logger;
+    }
+
+    /// <summary>Frontend tritt der Gruppe für einen bestimmten Agent bei</summary>
+    public async Task JoinAgentGroup(string agentId)
+    {
+        await Groups.AddToGroupAsync(Context.ConnectionId, $"agent-{agentId}");
+        _logger.LogDebug("Client {ConnectionId} joined group agent-{AgentId}", Context.ConnectionId, agentId);
+    }
+
+    /// <summary>Frontend verlässt die Gruppe für einen Agent</summary>
+    public async Task LeaveAgentGroup(string agentId)
+    {
+        await Groups.RemoveFromGroupAsync(Context.ConnectionId, $"agent-{agentId}");
+    }
+
+    public override async Task OnConnectedAsync()
+    {
+        _logger.LogInformation("SignalR client connected: {ConnectionId}", Context.ConnectionId);
+        await base.OnConnectedAsync();
+    }
+
+    public override async Task OnDisconnectedAsync(Exception? exception)
+    {
+        _logger.LogInformation("SignalR client disconnected: {ConnectionId}", Context.ConnectionId);
+        await base.OnDisconnectedAsync(exception);
+    }
+}