On 1.x-to-2.0 upgrade the /data/ai/venv already existed with stale
packages, so the entrypoint skipped the cp from /opt/venv and 2.0
Python tools failed with ImportError (#85).
Dockerfile now writes a SHA-256 of `pip freeze` to /opt/venv/.venv-version
at build time. The entrypoint compares that stamp against the copy on the
volume; on mismatch it does a clean nuke + re-copy and resets
installed.json so the UI correctly shows bundles as needing reinstall.
Models in /data/ai/models survive the refresh, so bundle reinstall only
reruns pip (model downloads are idempotent).
Backward compatible: if the image has no stamp (old build), the
entrypoint falls through to the existing skip-if-exists behavior.