Add a "Last Rendered" view to the webapp.
The Manager now stores (in the database) which job was the last
recipient of a rendered image, and serves that to the appropriate
OpenAPI endpoint.
A new SocketIO subscription + accompanying room makes it possible for
the web interface to receive all rendered images (if they survive the
queue, which discards images when it gets too full).
It doesn't update automatically yet (still needs SocketIO notifications),
and there is no check yet for whether there is actually any last-rendered
image at all, but at least there is a component that shows the image.
