Move backup execution out of the management command into a reusable
backup runner service that can execute an existing BackupRun record.
Add queue primitives and a run_pobsync_worker command so manual backup
requests can be recorded as queued SQL state and processed outside the
web request path.
Add a worker Docker service and pobsync worker CLI alias, with tests for
queued run creation, worker execution, manual run typing, and command
mapping.
