Add explicit Django model fields for global and host backup settings,
including SSH, rsync, source, excludes, and retention configuration.
Populate them from legacy JSON during migration, make the config
repository prefer structured fields, and update import/admin/tests around
the SQL-first configuration model.
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.
