修复首页热门信息为空问题并增加历史兜底

变更项：

1. 保留热门用户/热门群组/热门插件逻辑，未移除任何热门模块。

2. 为热门数据增加兜底策略：最近N天无数据时自动回退到全量历史Top5。

3. 覆盖插件、用户、群组三类热门榜单，避免首页出现整块空表。

db/stats_db.py

@@ -533,7 +533,7 @@ class StatsDBOperator(BaseDBOperator):
             latency_score = max(0, min(100, ((5000 - avg_response_time) / 4500) * 100))
         group_health_score = (success_rate * 0.7) + (latency_score * 0.3)
         
-        # 7. 最常用的插件
+        # 7. 最常用的插件（优先统计窗口，无数据时回退全量）
         top_plugins_sql = """
         SELECT plugin_name, SUM(total_calls) as total_calls
         FROM t_plugin_stats
@@ -543,8 +543,17 @@ class StatsDBOperator(BaseDBOperator):
         LIMIT 5
         """
         top_plugins = self.execute_query(top_plugins_sql, (start_date_str,)) or []
-        
-        # 8. 最活跃的用户
+        if not top_plugins:
+            top_plugins_all_sql = """
+            SELECT plugin_name, SUM(total_calls) as total_calls
+            FROM t_plugin_stats
+            GROUP BY plugin_name
+            ORDER BY total_calls DESC
+            LIMIT 5
+            """
+            top_plugins = self.execute_query(top_plugins_all_sql) or []
+
+        # 8. 最活跃的用户（优先统计窗口，无数据时回退全量）
         top_users_sql = """
         SELECT user_id, SUM(total_calls) as total_calls
         FROM t_user_stats
@@ -554,8 +563,17 @@ class StatsDBOperator(BaseDBOperator):
         LIMIT 5
         """
         top_users = self.execute_query(top_users_sql, (start_date_str,)) or []
-        
-        # 9. 最活跃的群组
+        if not top_users:
+            top_users_all_sql = """
+            SELECT user_id, SUM(total_calls) as total_calls
+            FROM t_user_stats
+            GROUP BY user_id
+            ORDER BY total_calls DESC
+            LIMIT 5
+            """
+            top_users = self.execute_query(top_users_all_sql) or []
+
+        # 9. 最活跃的群组（优先统计窗口，无数据时回退全量）
         top_groups_sql = """
         SELECT group_id, SUM(total_calls) as total_calls
         FROM t_group_stats
@@ -565,6 +583,15 @@ class StatsDBOperator(BaseDBOperator):
         LIMIT 5
         """
         top_groups = self.execute_query(top_groups_sql, (start_date_str,)) or []
+        if not top_groups:
+            top_groups_all_sql = """
+            SELECT group_id, SUM(total_calls) as total_calls
+            FROM t_group_stats
+            GROUP BY group_id
+            ORDER BY total_calls DESC
+            LIMIT 5
+            """
+            top_groups = self.execute_query(top_groups_all_sql) or []
         
         # 返回汇总数据
         return {