Add real-time dashboard updates with SignalR

Integrate SignalR to provide real-time dashboard update notifications.
- Added DashboardsHub and DashboardChangeNotifier on the backend.
- Modified SqlDashboardStorage to trigger notifications on changes.
- Registered SignalR services and mapped the hub endpoint.
- Updated Blazor clients to connect to the hub and refresh dashboards on change.
- Added SignalR client packages and necessary DI/configuration.

DbFirst.API/Dashboards/DashboardChangeNotifier.cs

@@ -0,0 +1,19 @@
+using DbFirst.API.Hubs;
+using Microsoft.AspNetCore.SignalR;
+
+namespace DbFirst.API.Dashboards;
+
+public class DashboardChangeNotifier : IDashboardChangeNotifier
+{
+    private readonly IHubContext<DashboardsHub> _hubContext;
+
+    public DashboardChangeNotifier(IHubContext<DashboardsHub> hubContext)
+    {
+        _hubContext = hubContext;
+    }
+
+    public void NotifyChanged()
+    {
+        _ = _hubContext.Clients.All.SendAsync("DashboardsChanged");
+    }
+}