Implement NotifyOnStatusChange feature for nodes and enhance Telegram notifications

- Added `NotifyOnStatusChange` property to the `Node` class and related DTOs to allow individual node configuration for status change notifications.
- Updated `NodeHealthCheckService` to send Telegram notifications to admins when a node's status changes, based on the new property.
- Enhanced the `ITelegramNotifier` interface with a method for notifying admins about node status changes.
- Modified the frontend to include a checkbox for `NotifyOnStatusChange` in the node editing dialog, allowing admins to easily configure this setting.
- Updated API documentation to reflect the new `notifyOnStatusChange` parameter in the node update endpoint.
- Added tests to ensure the correct behavior of the new feature and its integration with existing functionality.

docs/domain-model.md

@@ -48,6 +48,7 @@ AppUser
 | `Location`       | `string?`       | Страна/город/тег для выбора пользователем                    |
 | `Status`         | `NodeStatus`    | `Online` / `Offline` / `Unknown`                             |
 | `IsEnabled`      | `bool`          | Выключена админом → скрыта из самообслуживания                |
+| `NotifyOnStatusChange` | `bool`    | Слать админам в Telegram при каждом переходе Online↔Offline (см. NodeHealthCheckService); по умолчанию `false` |
 | `LastSyncAt`     | `DateTimeOffset?` | Последняя успешная синхронизация                            |
 | `CreatedAt`      | `DateTimeOffset`|                                                              |