fix(worker): prevent panic when jobs are pushed during graceful shutdown

Pool.Shutdown() closed the job queue channel, but jobs can still be
pushed while workers are draining: the feed scheduler ticker goroutine
is never cancelled, and the UI and API refresh handlers push from
detached goroutines that outlive the HTTP server shutdown. Any of them
sending on the closed queue panicked the process mid-shutdown.

Keep the queue channel open and instead close a dedicated shutdown
channel, guarded by sync.Once. Push now selects between delivering a
job and the shutdown signal, discarding jobs once shutdown begins, and
workers select between the queue and the shutdown signal, so they
still finish their current job before Shutdown returns.