Bookability: run date refresh synchronously, update timestamp immediately

Backend now awaits the refresh in a thread executor instead of queuing a
background task, so the response returns only after data is written. Frontend
drops the 3s setTimeout and invalidates the cache immediately on success.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/Bookability.tsx

@@ -250,11 +250,8 @@ const Bookability: React.FC = () => {
       return res.data
     },
     onSuccess: () => {
-      // Refetch after a short delay to allow background task to complete
-      setTimeout(() => {
-        queryClient.invalidateQueries({ queryKey: ['rate-matrix'] })
-        setRefreshingDate(null)
-      }, 3000)
+      queryClient.invalidateQueries({ queryKey: ['rate-matrix'] })
+      setRefreshingDate(null)
     },
     onError: () => setRefreshingDate(null),
   })