- Add cista/util/diskspace.py with MIN_FREE_BYTES limit and cached
check_free_space() helper.
- Check available space in File.write() before ftruncate/write.
- Catch ENOSPC in File.write and re-raise as InsufficientStorageError.
- upload_file_chunk catches both proactive and ENOSPC errors and
returns HTTP 507 Insufficient Storage.
- Add tests for low-disk rejection and ENOSPC handling.
