fix(HEL-494): 盘中双免费源故障切换并禁止问天假0覆盖

主源东财失败后自动改走腾讯行情，成功结果写入缓存；两源都失败时返回最近真实快照并标明延迟，不再显示假0。

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

backend/data/providers/tushare_dashboard.py

@@ -293,7 +293,10 @@ class DashboardMixin:
             last_error = str(exc)
         code_list = [item for item in str(codes or "").split(",") if item]
         try:
-            quotes = aggregator.tencent_market_quotes(code_list, expected_date=trade_date)
+            if code_list:
+                quotes = aggregator.tencent_stock_quotes(code_list, expected_date=trade_date)
+            else:
+                quotes = aggregator.tencent_market_quotes(code_list, expected_date=trade_date)
         except Exception as exc:
             raise TushareError(
                 f"eastmoney={last_error or 'empty'}；tencent={exc}"