Implement extension request and gift functionalities in billing system

- Added new endpoints for creating and managing extension requests, allowing users to request billing period extensions.
- Implemented admin approval processes for extension requests via Telegram, including inline buttons for approval and rejection.
- Introduced a gifting feature for admins to grant additional billing days directly to users without a request.
- Updated the support ticket model to accommodate extension requests and their associated properties.
- Enhanced the Telegram notifier to inform admins of new extension requests and notify users of approval or rejection.
- Updated frontend components to support the new extension request and gifting functionalities, including user interfaces for managing these features.
- Revised API documentation to reflect the new endpoints and their usage in the billing context.

backend/src/PnvPanel.Application/Common/Interfaces/ITelegramNotifier.cs

@@ -62,4 +62,14 @@ public interface ITelegramNotifier
         int amount,
         CancellationToken cancellationToken
     );
+
+    /// <summary>Заявка на продление оплаченного периода — инлайн-кнопки «Одобрить/Отклонить», решается
+    /// полностью в Telegram (аналогично заявке на роль).</summary>
+    Task NotifyAdminsExtensionRequestCreatedAsync(
+        Guid ticketId,
+        string userName,
+        int requestedDays,
+        string justification,
+        CancellationToken cancellationToken
+    );
 }