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/tests/PnvPanel.Application.Tests/Admin/Billing/RejectPaymentRequestCommandHandlerTests.cs

@@ -147,6 +147,8 @@ public class RejectPaymentRequestCommandHandlerTests
                 Arg.Any<DateTimeOffset?>(),
                 Arg.Any<CancellationToken>()
             );
+        // Фронт (/billing) должен узнать о смене статуса сразу, не дожидаясь опроса.
+        await _notifier.Received(1).NotifyBillingStatusChangedAsync(userId, Arg.Any<CancellationToken>());
     }
 
     [Fact]