fix: exchange-specific volume rank APIs for OKX and full top20

Co-authored-by: Cursor <cursoragent@cursor.com>

manual_trading_hub/hub.py

@@ -381,14 +381,18 @@ def _refresh_volume_ranks(*, force: bool = False) -> dict:
         if not ex_key or not ex.get("enabled"):
             continue
         resp = _fetch_instance_volume_rank_sync(ex, top_n=TOP_N_DEFAULT)
-        if resp.get("ok"):
+        if resp.get("ok") and resp.get("items"):
             cache = merge_exchange_rank(cache, ex_key, resp)
         else:
             msg = str(resp.get("msg") or resp.get("error") or "拉取失败")
+            if resp.get("ok") and not resp.get("items"):
+                msg = msg if msg != "拉取失败" else "无有效成交额数据"
             errors.append(f"{ex_key}:{msg}")
             exchanges = dict(cache.get("exchanges") or {})
             prev = dict(exchanges.get(ex_key) or {})
             prev["error"] = msg
+            if not prev.get("items"):
+                prev["items"] = []
             exchanges[ex_key] = prev
             cache["exchanges"] = exchanges
     cache["rank_date"] = expected