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>

config/README.md

@@ -12,6 +12,9 @@ These registries describe the approved product surface of the standalone applica
   providers, model entry points, CSS layers, and remaining code hotspots.
 - `data-fields.config.json`: canonical data products, provider eligibility, intended use, and
   known blocked datasets.
+- `datahub.config.json`: optional read-only client for `xiaobai-datahub`. Each dataset has its
+  own `read` / `shadow` flag, all default off. Environment variables `DATAHUB_READ_*` and
+  `DATAHUB_SHADOW_*` can override a single dataset without a master switch.
 - `data-quality.config.json`: freshness, coverage, units, adjustment, point-in-time, and
   fail-closed rules for every canonical data product.
 - `jobs.config.json`: background schedules, dependencies, lock keys, retry policy, timeouts,