feat: 增强管理员工作台聊天统计看板

- 工作台接入聊天消息总数、会话总数、活跃智能体数、趋势与 Top5 排行

- dashboard 接口新增 chatStatus 与聊天统计字段，分析库不可用时明确降级

- today 维度按小时聚合聊天趋势，并补充后端查询与测试覆盖

easyflow-starter/easyflow-starter-all/src/main/resources/db/migration/analyticaldb/V3__analyticaldb_dashboard_tenant.sql

@@ -0,0 +1,18 @@
+ALTER TABLE dws_chat_assistant_day
+    ADD COLUMN IF NOT EXISTS `tenant_id` UInt64 DEFAULT 0 AFTER `dimension_id`;
+
+ALTER TABLE dws_chat_assistant_day DELETE WHERE `tenant_id` = 0;
+
+INSERT INTO dws_chat_assistant_day
+SELECT
+    toDate(l.created) AS stat_date,
+    l.assistant_id AS dimension_id,
+    ifNull(s.tenant_id, toUInt64(0)) AS tenant_id,
+    uniqExact(l.session_id) AS session_count,
+    count() AS message_count
+FROM ods_chat_log l
+LEFT JOIN (
+    SELECT id, tenant_id
+    FROM ods_chat_session FINAL
+) s ON s.id = l.session_id
+GROUP BY stat_date, dimension_id, tenant_id;