Implement billing status notification and enhance user management integration

- Added `NotifyBillingStatusChangedAsync` method to `IRealtimeNotifier` for notifying clients about changes in billing status.
- Updated `BillingConfigResumer` to call the new notification method after modifying billing configurations, ensuring users receive real-time updates.
- Enhanced `ListUsersQueryHandler` to include a `BillingPendingReview` property in `UserSummaryDto`, indicating if a user has a pending payment request awaiting confirmation.
- Refactored various command handlers to utilize `AdvisoryLock` for managing concurrent requests, preventing race conditions in billing operations.
- Updated tests to cover new notification behaviors and ensure proper functionality in billing status management.

backend/src/PnvPanel.Api/Hubs/SignalRRealtimeNotifier.cs

@@ -151,4 +151,11 @@ internal sealed class SignalRRealtimeNotifier(IHubContext<PanelHub> hubContext)
             .Clients.Group(GroupNames.User(userId))
             .SendAsync("ticketUpdated", new { ticketId }, cancellationToken);
     }
+
+    public Task NotifyBillingStatusChangedAsync(Guid userId, CancellationToken cancellationToken)
+    {
+        return hubContext
+            .Clients.Group(GroupNames.User(userId))
+            .SendAsync("billingStatusChanged", new { userId }, cancellationToken);
+    }
 }