When FTP download of a 3MF fails (e.g. BambuStudio-initiated prints),
the fallback archive has file_path="". Path.exists() on
settings.base_dir / "" resolves to the base directory itself and
returns True, so ZipFile() then fails with [Errno 21] Is a directory.
Replace .exists() with .is_file() across all 15 archive route checks
and 1 in main.py. Add file_path truthiness guard for finish photo
capture to prevent saving photos under the base directory.
