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.

docs/api-design.md

@@ -362,6 +362,11 @@ totalConfigs, activeConfigs, totalUsedUpBytes, totalUsedDownBytes }` — счи
 | `newsPublished`        | `{ id, title, createdAt }`                                     | все (broadcast) |
 | `ticketCreated`        | `{ ticketId, userId, userName, type }`                          | `admins`     |
 | `ticketUpdated`        | `{ ticketId }`                                                  | владельцу    |
+| `billingStatusChanged` | `{ userId }`                                                    | владельцу    |
+
+`billingStatusChanged` — безадресный пинг без данных (см. `BillingConfigResumer` в
+[domain-model.md](domain-model.md#синхронизация-панели-3x-ui-со-статусом-оплаты--billingconfigresumer)):
+клиент в ответ инвалидирует `my-billing-status`, а не читает payload.
 
 ### Client → Server
 Клиент только слушает; группировка по пользователю происходит на сервере при подключении, по