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.Infrastructure/Storage/DiskFileStorage.cs

@@ -31,4 +31,13 @@ internal sealed class DiskFileStorage(IOptions<FileStorageOptions> options) : IF
 
         return Task.FromResult<Stream?>(File.OpenRead(path));
     }
+
+    public Task DeleteAsync(string storedFileName, CancellationToken cancellationToken)
+    {
+        var path = Path.Combine(options.Value.RootPath, storedFileName);
+        if (File.Exists(path))
+            File.Delete(path);
+
+        return Task.CompletedTask;
+    }
 }