fix(HEL-494): 数据中枢独占调度，主网站不再回退旧接口

主网站只向中枢要业务数据；来源选择、切源、补数全部在中枢内部完成，失败不再走东财/腾讯/Tushare 保底。

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

backend/data/gateway.py

@@ -38,7 +38,7 @@ class DataGateway:
         if dataset_id:
             self.policy.assert_allowed(dataset_id, "tushare", usage)
         legacy = self.tushare_provider.client()
-        legacy.realtime_aggregator = self.realtime_observer
+        legacy.realtime_aggregator = None
         return DatahubAwareTushareClient(legacy, self.datahub)
 
     def dataset_status(self, trade_date: str) -> list[dict[str, Any]] | None: