The gateway now follows the job each printer was handed. Started, finished
and failed are written to its SQLite store and sent as job_event on every
connection until a job_event_ack clears them, so an outcome reached during
an outage, or before a restart, is still delivered.
A start that fails after being accepted goes through the same outbox
instead of a one-shot message to whoever was connected. Nothing is
concluded from a printer's state until the job has been seen running,
because a printer told to start still reports the previous job's FINISH.
The backend still owns the queue; this only reports.
