Refactor SignalR hub registration and background tasks

- Use Forget() extension for fire-and-forget tasks in MgGridSignalRDataSource
- Remove legacy commented DevAdminSignalRHub code
- Switch to DynamicMethodRegistry for hub method registration, improving startup performance by deferring reflection
- Add clarifying comment for remaining legacy message handling code

AyCode.Blazor.Components/Components/Grids/MgGridSignalRDataSource.cs

@@ -138,7 +138,7 @@ public class MgGridSignalRDataSource<TDataItem, TId> : GridCustomDataSource
             _logger?.Debug($"[MgGridSignalRDataSource] Returning {localResult.Count} local items, refreshing in background");
             
             // Refresh in background (fire-and-forget)
-            _ = RefreshInBackgroundAsync(filterKey);
+            RefreshInBackgroundAsync(filterKey).Forget();
             
             return localResult;
         }