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/tests/test_configure_commands.py

@@ -6,7 +6,7 @@ from django.core.management import call_command
 from django.test import TestCase
 
 from pobsync_backend.config_source import DjangoConfigSource
-from pobsync_backend.models import GlobalConfig, HostConfig
+from pobsync_backend.models import GlobalConfig, HostConfig, ScheduleConfig
 
 
 class ConfigureCommandsTests(TestCase):
@@ -54,3 +54,19 @@ class ConfigureCommandsTests(TestCase):
         effective = DjangoConfigSource().effective_config_for_host("web-01")
         self.assertEqual(effective["retention"]["yearly"], 2)
         self.assertEqual(effective["excludes_effective"], ["/tmp/***"])
+
+    def test_configure_schedule_creates_sql_schedule(self) -> None:
+        host = HostConfig.objects.create(host="web-01", address="web-01.example.test")
+        out = StringIO()
+
+        call_command(
+            "configure_pobsync_schedule",
+            host.host,
+            cron="15 2 * * *",
+            prune=True,
+            stdout=out,
+        )
+
+        schedule = ScheduleConfig.objects.get(host=host)
+        self.assertEqual(schedule.cron_expr, "15 2 * * *")
+        self.assertTrue(schedule.prune)