Convert 'last-rendered' to sqlc. The query is slightly suboptimal.
There's a bug in sqlc: the `ON CONFLICT DO UPDATE` clause is generated
incorrectly. See https://github.com/sqlc-dev/sqlc/issues/3334 for more
info.
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).