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/Hubs/DashboardsHub.cs

@@ -0,0 +1,7 @@
+using Microsoft.AspNetCore.SignalR;
+
+namespace DbFirst.API.Hubs;
+
+public class DashboardsHub : Hub
+{
+}