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_client.py

@@ -66,3 +66,5 @@ class TushareClient(
     _stock_listing_lock: ClassVar[Lock] = Lock()
     _suspension_cache: ClassVar[dict[str, dict[str, str] | None]] = {}
     _suspension_lock: ClassVar[Lock] = Lock()
+    _sw_member_cache: ClassVar[dict[str, Any]] = {}
+    _sw_member_lock: ClassVar[Lock] = Lock()