(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/management/commands/run_pobsync_backup.py

@@ -30,7 +30,7 @@ class Command(BaseCommand):
         try:
             host = HostConfig.objects.get(host=host_name, enabled=True)
         except HostConfig.DoesNotExist as exc:
-            raise CommandError(f"Missing enabled HostConfig {host_name!r}") from exc
+            raise CommandError(f"Missing enabled host {host_name!r}") from exc
 
         run = BackupRun.objects.create(
             host=host,