feat: add niuone US market + X timeline sources on HF tip

Add direct EastMoney daily and board flow fallbacks

Prefer EastMoney daily fallback after Yahoo

Prioritize Yahoo daily K-line and improve financial sample coverage

Speed up futures basis and refresh API docs

Add ETF premium and HK short selling sources

Add fleur-derived data endpoints and source fallbacks

fix: 引入全局源调用池，防止 HF Pro tier worker pool 饱和

docs: 更新基差接口文档 - 补充日内走势和返回结构说明

docs: 更新股指期货基差接口文档（支持多日查询）

docs: 添加股指期货基差接口文档

docs: 添加 purge-all 接口文档

docs: update fund flow source rotation in API docs

docs: add start_year/end_year params to financial endpoint docs

Add ETF realtime quote endpoint with source rotation

Apply fixes: search SSE streaming, fund-flow fallback, default search config