/healthz proves the process answers. It stays green when headless Chrome is
wedged, out of memory or missing after a bad build, which on a public status
page is worse than no status page.
/healthz/render renders a real page and returns 503 when that fails. Success
means bytes and not flagged blank, because a renderer producing white
images is broken.
It is fenced so it cannot be used as a free renderer or a load lever: at most one real render a minute, everyone else gets the cached last result with its age attached, queued behind paying customers, never metered.