fix: ajout méthode is_stale() manquante dans InvertedIndex

backend/search.py

@@ -361,6 +361,10 @@ class InvertedIndex:
         self._sorted_tokens: "SortedList" = SortedList()
         self._ready: bool = False  # True after initial build
 
+    def is_stale(self) -> bool:
+        """Return True if the index has not been built yet."""
+        return not self._ready
+
     def rebuild(self) -> None:
         """Rebuild inverted index from the global ``index`` dict.