Make dashboard change notifications async and robust

Refactored DashboardChangeNotifier and IDashboardChangeNotifier to use async notification with improved error handling and logging. Updated SqlDashboardStorage to call the new async notification method after dashboard changes, ensuring non-blocking and reliable client updates.

DbFirst.API/Dashboards/IDashboardChangeNotifier.cs

@@ -2,5 +2,5 @@ namespace DbFirst.API.Dashboards;
 
 public interface IDashboardChangeNotifier
 {
-    void NotifyChanged();
+    Task NotifyChangedAsync();
 }