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/realtime.py

@@ -236,6 +236,15 @@ class WebRealtimeAggregator:
         self,
         codes: list[str],
         expected_date: str = "",
+    ) -> list[dict[str, Any]]:
+        quotes = self.tencent_stock_quotes(codes, expected_date="", minimum=200)
+        return self._filter_quotes_by_date(quotes, expected_date)
+
+    def tencent_stock_quotes(
+        self,
+        codes: list[str],
+        expected_date: str = "",
+        minimum: int | None = None,
     ) -> list[dict[str, Any]]:
         symbols: list[str] = []
         seen: set[str] = set()
@@ -280,10 +289,11 @@ class WebRealtimeAggregator:
                     quotes.extend(rows)
                 elif not status.get("ok"):
                     errors.append(str(status.get("error") or "batch failed"))
-        if len(quotes) < 200:
+        floor = minimum if minimum is not None else max(1, int(len(symbols) * 0.5))
+        if len(quotes) < floor:
             detail = f"；{'; '.join(errors[:3])}" if errors else ""
             raise RealtimeAggregateError(
-                f"Tencent market snapshot too small: {len(quotes)}{detail}"
+                f"Tencent quotes too small: {len(quotes)}/{len(symbols)}{detail}"
             )
         return self._filter_quotes_by_date(quotes, expected_date)