test docker-compose.yml in workflows #670

Refs #597

docker-compose.yml isn't exercised by any workflow. Renovate keeps updating its image tags (e.g. #653, #656), but nothing confirms the stack still starts after those updates.

Goal

A pull request workflow job that:

• builds what the stack needs and brings it up with docker compose up --wait (or similar)
• smoke-tests it: the API health endpoint responds, and the frontend serves its page and proxies /api to the API
• always tears it down with docker compose down -v
• uses no secrets, only defaults or dummy test values
• is listed in PR Results' needs, so a broken compose stack blocks the merge