[edalzell]

Pages are tracked under the domain they were requested from, but invalidation resolves URLs in the current process. In a queue worker there's no request, so a relative site URL (/) resolves against APP_URL; if that differs at all from the real origin (e.g. http://localhost vs https://www.example.com), nothing is found and the page stays stale. sync only works because the CP request happens to share the frontend's origin.

Each site now records the domains it has been cached under (static-cache:site-domains), and invalidating a URL also checks every domain recorded for that URL's site. Other sites' domains are left alone. Background recache still only warms the domain the URL resolves to; matches under other recorded domains are invalidated instead, since those hosts come from request headers and shouldn't receive the recache token.

Closes #3291