fix(HEL-494): enforce datahub as sole website market boundary

Co-authored-by: multica-agent <github@multica.ai>

backend/data/gateway.py

@@ -1,6 +1,5 @@
 from __future__ import annotations
 
-from collections.abc import Callable
 from dataclasses import dataclass
 from datetime import datetime
 from typing import Any
@@ -95,11 +94,8 @@ class DataGateway:
 
 def build_data_gateway(
     credentials: dict[str, object],
-    tushare_token_supplier: Callable[[], str] | None = None,
     datahub_settings: DatahubSettings | None = None,
 ) -> DataGateway:
-    # Website Tushare tokens are not used to assemble market clients.
-    del tushare_token_supplier
     policy = DataSourcePolicy.load()
     settings = datahub_settings or DatahubSettings.load(credentials=credentials)
     datahub_client = DatahubClient(settings)