Update the scheduler to reflect the actual scheduled BackupRun status after
a run completes, so prune warnings are shown as schedule warnings instead
of being reported as successful schedule executions.
Treat SQL-backed Django models as the source of truth for pobsync
configuration, exporting runtime YAML only as a compatibility layer for
the existing engine. Add a database-driven scheduler command, Docker
scheduler services, schedule run-state fields, and tests for scheduler,
config export, and retention behavior.
