Add admin maintenance endpoints and file deletion functionality

- Introduced a new `/api/admin/maintenance` route for administrative maintenance tasks, requiring admin authorization.
- Implemented the `DeleteAsync` method in `IFileStorage` to allow for the deletion of files associated with closed support tickets.
- Updated API documentation to include details about the new maintenance operations and their effects on closed tickets.
- Enhanced frontend routing to include the new maintenance section in the admin panel, improving navigation for administrators.
- Added localization support for maintenance-related actions in both Russian and English.

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

@@ -12,4 +12,7 @@ public interface IFileStorage
 
     /// <summary>Null, если файла с таким именем нет на диске (например, удалён вручную).</summary>
     Task<Stream?> OpenReadAsync(string storedFileName, CancellationToken cancellationToken);
+
+    /// <summary>Идемпотентно — отсутствие файла не ошибка (мог быть удалён вручную).</summary>
+    Task DeleteAsync(string storedFileName, CancellationToken cancellationToken);
 }