feat(HEL-402): 接通网站首批只读 datahub 并建立双路对比

默认全部读取/影子开关关闭，网站继续走旧 Tushare 链路；开启单项时只替换该类原料并在失败时回旧，问天保持旧路径。

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

backend/data/datahub/__init__.py

@@ -0,0 +1,15 @@
+from backend.data.datahub.bridge import DatahubAwareTushareClient, DatahubBridge
+from backend.data.datahub.client import DatahubClient, DatahubResponse
+from backend.data.datahub.errors import DatahubError
+from backend.data.datahub.settings import DATASETS, DatahubSettings, DatasetFlags
+
+__all__ = [
+    "DATASETS",
+    "DatahubAwareTushareClient",
+    "DatahubBridge",
+    "DatahubClient",
+    "DatahubError",
+    "DatahubResponse",
+    "DatahubSettings",
+    "DatasetFlags",
+]