fix: add missing Node import in FileService

- Import OCP\Files\Node for proper type resolution
- Fix Psalm docblock type annotations
- Use fully qualified namespace in return types

Signed-off-by: Vitor Mattos <1079143+vitormattos@users.noreply.github.com>

Author: vitormattos

lib/Service/FileService.php

@@ -31,6 +31,7 @@
 use OCP\AppFramework\Db\DoesNotExistException;
 use OCP\Files\IMimeTypeDetector;
 use OCP\Files\IRootFolder;
+use OCP\Files\Node;
 use OCP\Files\NotFoundException;
 use OCP\Http\Client\IClientService;
 use OCP\IAppConfig;