(refactor) Normalize runtime config labels

Hide the old pobsync_home field from the Django admin and replace legacy
operator-facing labels with runtime state root and backup root terminology.

Rename admin compatibility fieldsets, update self-check/config-check text,
and refresh management command help so Django/systemd stays the primary
mental model.

src/pobsync_backend/management/commands/configure_pobsync_global.py

@@ -18,7 +18,7 @@ class Command(BaseCommand):
     def add_arguments(self, parser) -> None:
         parser.add_argument("--name", default="default")
         parser.add_argument("--backup-root", required=True)
-        parser.add_argument("--pobsync-home", default=settings.POBSYNC_HOME)
+        parser.add_argument("--pobsync-home", default=settings.POBSYNC_HOME, help="Runtime state root")
         parser.add_argument("--ssh-user", default="root")
         parser.add_argument("--ssh-port", type=int, default=22)
         parser.add_argument("--source-root", default="/")