leaderboard: catch only network errors (RequestException), not all exceptions

app.py

@@ -126,12 +126,14 @@ def leaderboard() -> pd.DataFrame:
     joined from the task registry at read time; if that fetch fails the board
     renders empty rather than showing a wrong ranking.
     """
+    from requests.exceptions import RequestException
+
     try:
         df = _read_results()
         datasets = manifest_ids(fetch_manifest(TOKEN))
         registry = scoreable_tasks(fetch_tasks_registry(TOKEN), datasets)
         by_id = {_norm_id(task["task_id"]): task for task in registry}
-    except Exception:
+    except RequestException:
         return pd.DataFrame(columns=BASE_COLUMNS)
     required = set(by_id)
     if df.empty or not required: