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.
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.
Add a Django admin-backed management layer for pobsync configs, runs,
snapshots, and schedules. Keep the existing CLI engine as the execution
source of truth, add import/run management commands, and provide SQLite
default plus optional MariaDB Docker Compose support.
