(refactor) Normalize maintainer command labels

Prefer --schedule-expression for scripted schedule updates while keeping
--cron as a compatibility alias.

Clean up management command help, errors, and output so operator-facing
text talks about hosts, global config, and Django backup configuration
instead of model names or old SQL-backed pobsync wording.

src/pobsync_backend/tests/test_configure_commands.py

@@ -23,7 +23,7 @@ class ConfigureCommandsTests(TestCase):
         config = GlobalConfig.objects.get(name="default")
         self.assertEqual(config.backup_root, "/backups")
         self.assertEqual(config.retention_daily, 3)
-        self.assertIn("Created GlobalConfig", out.getvalue())
+        self.assertIn("Created global config", out.getvalue())
 
     def test_configure_host_uses_global_retention_defaults(self) -> None:
         GlobalConfig.objects.create(
@@ -61,7 +61,7 @@ class ConfigureCommandsTests(TestCase):
         call_command(
             "configure_pobsync_schedule",
             host.host,
-            cron="15 2 * * *",
+            schedule_expression="15 2 * * *",
             prune=True,
             stdout=out,
         )