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-ui-admin/app/src/api/dashboard.ts

@@ -8,30 +8,45 @@ export interface DashboardOverviewQuery {
 
 export interface DashboardSummary {
   activeUserTotal: number;
+  activeAssistantTotal: number;
   botTotal: number;
+  chatMessageTotal: number;
+  chatSessionTotal: number;
   knowledgeBaseTotal: number;
   userTotal: number;
   workflowTotal: number;
 }
 
+export interface DashboardChatStatus {
+  available: boolean;
+  message: string;
+}
+
 export interface DashboardTrendItem {
   activeUserTotal: number;
+  chatMessageTotal: number;
+  chatSessionTotal: number;
   key: string;
   label: string;
 }
 
 export interface DashboardDistributionItem {
   activeUserTotal: number;
+  assistantId?: number | string;
+  avgMessagePerSession?: number;
   botTotal: number;
   key: string;
   knowledgeBaseTotal: number;
   label: string;
+  messageTotal?: number;
+  sessionTotal?: number;
   userTotal: number;
   value: number;
   workflowTotal: number;
 }
 
 export interface DashboardOverviewResponse {
+  chatStatus: DashboardChatStatus;
   distribution: DashboardDistributionItem[];
   query: DashboardOverviewQuery;
   summary: DashboardSummary;