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/trend.py

@@ -243,6 +243,7 @@ class HeavenTrendMixin:
                 "detail": (
                     f"申万二级 {sector.get('name') or '--'} {sector.get('code') or '--'} "
                     f"成分覆盖 {int(sector.get('quote_count') or 0)}/{int(sector.get('member_count') or 0)}"
+                    + ("；延迟快照" if sector.get("delayed") or sector.get("delay_notice") else "")
                 ),
             },
             {
@@ -341,7 +342,9 @@ class HeavenTrendMixin:
             issues.append("行业外显缺少申万官方行情")
         if sector and sector_coverage_issue:
             issues.append(sector_coverage_issue)
-        if sector.get("realtime") and not sector.get("relative_turnover"):
+        if sector.get("delay_notice"):
+            issues.append(str(sector.get("delay_notice")))
+        if sector.get("realtime") and not sector.get("relative_turnover") and not sector.get("delayed"):
             issues.append("行业内核缺少相对全市场换手活跃度")
 
         stock = stock or {}