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.
Introduce a ConfigSource interface so scheduled backups no longer need
to load host configuration directly from runtime YAML. Add a Django-backed
config source for SQL-driven backup runs, keep file-based config as the
CLI default, and make scheduled prune execution actually apply retention
after successful runs.
