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/features/heaven/market_context.py

@@ -299,6 +299,14 @@ class HeavenMarketContextMixin:
             and not cached.get("realtime")
             and int(cached.get("schema_version") or 0) >= 6
         )
+        cached_quotes = int((cached or {}).get("quote_count") or 0)
+        cached_lkg = bool(
+            cached
+            and cached_date == trade_date
+            and cached.get("taxonomy") == "sw_l2"
+            and cached_quotes > 0
+            and int(cached.get("schema_version") or 0) >= 6
+        )
         if market_mode != "intraday" and cached_valid:
             return cached
         if not self.configured:
@@ -311,8 +319,12 @@ class HeavenMarketContextMixin:
                 allow_realtime_close=market_mode == "closed",
             )
         except TushareError as exc:
-            if cached_valid:
-                return cached
+            if cached_lkg:
+                delayed = dict(cached)
+                delayed["delayed"] = True
+                delayed["delay_notice"] = "主备免费行情均暂不可用，显示最近一次真实快照"
+                delayed["realtime"] = market_mode == "intraday"
+                return delayed
             return {
                 "name": "",
                 "code": "",
@@ -323,12 +335,16 @@ class HeavenMarketContextMixin:
                 "precise": False,
                 "inner_precise": False,
                 "outer_precise": False,
-                "coverage": 0,
-                "member_count": 0,
-                "quote_count": 0,
                 "error": f"申万二级行业数据获取失败：{exc}",
             }
-        if not payload.get("realtime") and payload.get("precise"):
+        if int(payload.get("quote_count") or 0) > 0:
+            self.database.save_data_snapshot(
+                "heaven_sector",
+                cache_key,
+                str(payload.get("source") or "tushare"),
+                payload,
+            )
+        elif not payload.get("realtime") and payload.get("precise"):
             self.database.save_data_snapshot(
                 "heaven_sector",
                 cache_key,