(refactor) Remove YAML config import and export path

Drop the pre-Django YAML import/export management commands and remove the
file-based config loader fallback from the backup and retention engines.

Keep the runtime config bridge backed by Django models, and add tests that
ensure engine operations require an explicit Django config source.

docs/development.md

@@ -96,23 +96,6 @@ The updater is intentionally a small wrapper around the installer for routine pr
 non-interactive, preserve the existing environment file, skip OS package installation, skip superuser creation, and still
 run the Django/runtime refresh steps needed after a code update.
 
-## Migration Helpers
-
-Import pre-Django YAML configs during a one-time migration:
-
-```
-python3 manage.py import_pobsync_configs --prefix /opt/pobsync
-```
-
-Export SQL config back to YAML for inspection or one-off compatibility:
-
-```
-python3 manage.py export_pobsync_configs --prefix /opt/pobsync
-```
-
-These commands are migration helpers, not the normal operating model. After import, review and continue operating from
-the Django control panel.
-
 ## Docker With SQLite
 
 Docker Compose is useful for local development and disposable test installs. Native systemd is preferred for production
@@ -192,4 +175,3 @@ Next refactor targets:
 
 - Move more snapshot lifecycle details into typed domain objects.
 - Replace remaining dictionary-shaped config at engine boundaries.
-- Remove YAML migration import/export once production migration no longer needs it.