fix(updater): add snapshot fallback for source backups

[xiami762]

Summary

• keep the existing direct tar.gz backup as the primary path
• fall back to archiving a filtered temporary source snapshot when direct backup fails
• write backups through a partial file and clean up failed archives and temporary snapshots
• keep runtime data, Python environments, and Node dependencies out of fallback backups

Root cause

The updater can archive the live source tree while files are changing on Windows. Python tarfile records a file size before reading it, so a file that shrinks during the read can raise unexpected end of data and abort the upgrade.

Tests

• uv run ruff check flocks/updater/updater.py tests/updater/test_updater.py
• uv run pytest -q tests/updater/test_updater.py tests/updater/test_restart_handoff.py