Fix DISABLE_PERIODIC not suppressing periodic dispatch

Flag was read and used to skip credentials_checks but the run_due call
was never gated on it, so periodics ran regardless.

room_cal/_health.py

@@ -102,7 +102,7 @@ async def probe_once() -> None:
       },
       'checked_at': datetime.now(timezone.utc).isoformat(),
     }
-    if store_ok and creds_ok:
+    if store_ok and creds_ok and not periodic_disabled:
       from room_cal._periodic import run_due, _register_all_periodics
       _register_all_periodics()
       await run_due(api)