upgrade-backend-deps

Upgrade Python backend dependencies to their latest compatible versions. Resolves the lockfile with uv lock --upgrade, syncs the venv, bumps the matching >= pins in backend/pyproject.toml and ruff-pre-commit rev in .pre-commit-config.yaml, runs the backend unit tests, rebuilds the backend Docker image if requested, verifies thread state still loads on the restarted stack, opens a PR with the change (watching CI for…