fix(HEL-487): 盘中当天看板在 rt_k 无权限时降级到免费实时源

rt_k 失败、无权限、超时或空结果时改用东财全市场快照，再失败则用腾讯批量行情；两者都失败仍不退回昨天。

Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: multica-agent <github@multica.ai>

backend/data/gateway.py

@@ -37,7 +37,9 @@ class DataGateway:
     ) -> TushareClient:
         if dataset_id:
             self.policy.assert_allowed(dataset_id, "tushare", usage)
-        return DatahubAwareTushareClient(self.tushare_provider.client(), self.datahub)
+        legacy = self.tushare_provider.client()
+        legacy.realtime_aggregator = self.realtime_observer
+        return DatahubAwareTushareClient(legacy, self.datahub)
 
     def dataset_status(self, trade_date: str) -> list[dict[str, Any]] | None:
         return self.datahub.dataset_status(trade_date)