venv cannot be shipped in the repo — it embeds absolute paths and
distro-specific compiled wheels (asyncpg, uvloop, cryptography), so it
dies on any other machine. Instead, when the host has no python >= 3.11
(Ubuntu 18.04: 3.6, 22.04: 3.10; system python must not be replaced),
install.sh fetches a standalone astral python-build-standalone CPython
3.12 into .python/ — self-contained, needs only glibc >= 2.17, works
on any distro. The tarball is cached next to the repo and reused; on
offline servers a pre-downloaded tarball next to install.sh wins.
Verified locally: tarball layout, venv creation, pip install.