The #666 pytest gate's 'Install uv' step used `pip install uv`, which fails
on the ubuntu-latest runner with PEP 668 externally-managed-environment →
the test job errored → build+deploy skipped (deploy blocked). Switch to the
official standalone installer (curl astral.sh/uv/install.sh) + $GITHUB_PATH.
pytest itself is green locally (1087 passed); this was purely the runner uv
install. Unblocks deploy-tradein.yml.