Restore 5s calendar polling for near-real-time shared event sync

Reverts the AW-3 optimization that increased polling from 5s to 30s.
The faster interval is needed for shared calendar edits and invited
editor changes to appear promptly on other users' views.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/calendar/CalendarPage.tsx

@@ -229,9 +229,8 @@ export default function CalendarPage() {
       });
       return data;
     },
-    // AW-3: Reduce from 5s to 30s — personal organiser doesn't need 12 calls/min
+    refetchInterval: 5_000,
-    refetchInterval: 30_000,
+    staleTime: 5_000,
-    staleTime: 30_000,
   });
 
   const selectedEvent = useMemo(