Path traversal in get_exec_file_path escapes the exec_file sandbox

Hey! Found that get_exec_file_path in backend/util/file.py joins the user-controlled path with no containment check, so ../../evil.txt or an absolute path like /etc/passwd escapes the exec_file sandbox. Media blocks pass graph input straight through to it. Happy to PR a fix (resolve + reject outside base) if that helps.