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.Application/Common/Interfaces/IIdentityService.cs

@@ -36,7 +36,11 @@ public sealed record UserSummaryDto(
     bool IsBlocked,
     DateTimeOffset? ActivatedAt,
     bool BillingEnabled,
-    DateTimeOffset? BillingPaidUntil
+    DateTimeOffset? BillingPaidUntil,
+    /// <summary>Есть Subscription-заявка на оплату в AwaitingConfirmation — конфиги не гасятся, пока
+    /// админ не решит (см. BillingService). Заполняется в ListUsersQueryHandler (не здесь — Identity
+    /// не должен знать про PaymentRequests), false по умолчанию для мест, не подгружающих это поле.</summary>
+    bool BillingPendingReview = false
 );
 
 public sealed record UserStatsDto(int Total, int Activated);