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.Api/Program.cs

@@ -191,6 +191,7 @@ app.MapAdminAppEndpoints();
 app.MapAdminNewsEndpoints();
 app.MapSupportEndpoints();
 app.MapAdminSupportEndpoints();
+app.MapAdminMaintenanceEndpoints();
 app.MapTelegramEndpoints();
 
 app.MapHub<PanelHub>("/hubs/panel");