refactor: replace legacy CLI with Django command surface

Retire the old YAML and cron oriented pobsync CLI commands and expose a
SQL-first Django-backed command surface instead. Add schedule and
retention management commands, move shared defaults/parsing out of legacy
commands, remove obsolete command modules, and update documentation and
tests for the new workflow.

src/pobsync_backend/management/commands/configure_pobsync_global.py

@@ -6,8 +6,8 @@ from typing import Any
 from django.conf import settings
 from django.core.management.base import BaseCommand, CommandError
 
-from pobsync.cli import parse_retention
-from pobsync.commands.install import DEFAULT_EXCLUDES, DEFAULT_RSYNC_ARGS
+from pobsync.config.retention import parse_retention
+from pobsync.config.defaults import DEFAULT_EXCLUDES, DEFAULT_RSYNC_ARGS
 from pobsync.util import is_absolute_non_root
 from pobsync_backend.models import GlobalConfig