(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/run_pobsync_scheduler.py

@@ -18,7 +18,7 @@ class Command(BaseCommand):
     help = "Run due pobsync schedules from the Django database."
 
     def add_arguments(self, parser) -> None:
-        parser.add_argument("--prefix", default=settings.POBSYNC_HOME, help="Pobsync home directory")
+        parser.add_argument("--prefix", default=settings.POBSYNC_HOME, help="Runtime state root")
         parser.add_argument("--once", action="store_true", help="Check once and exit")
         parser.add_argument("--loop", action="store_true", help="Keep checking schedules")
         parser.add_argument("--interval", type=int, default=60, help="Loop interval in seconds")