After the venv is created, $PY still points to the Homebrew/system
interpreter, which triggers PEP 668 (externally-managed-environment)
on modern macOS. Introduce VENV_PY pointing at the venv's own
interpreter and use it for all pip installs and the final uvicorn
launch.
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
