feat: 前端指标体系增强 — Dashboard/Metrics API、2×4 统计区、趋势图面积+异常标记、连续状态列

- 新增 GET /api/dashboard 合并原 summary+targets 首屏接口
- 新增 GET /api/targets/:id/metrics 合并原 stats+trend 概览接口
- 后端指标纯函数：可用率、百分位、故障段分析、连续状态、UTC 小时分桶
- ProbeStore 窗口取数方法替代全量历史查询
- SummaryCards 扩展为 4 卡片（新增异常事件数）+ 数据新鲜度展示
- 表格新增「连续」列（Tag 渲染 capped 状态）
- OverviewTab 重构为 2×4 Statistic 多维度布局
- TrendChart 改为延迟范围面积图 + 红色异常标记点
- 删除旧路由（summary/targets/trend）和 computeTrendStats
- 同步 delta specs 到主 specs 并归档变更

src/shared/api.ts

@@ -20,6 +20,28 @@ export interface CheckResult {
   timestamp: string;
 }
 
+export interface CurrentStreak {
+  capped?: boolean;
+  count: number;
+  up: boolean;
+}
+
+export interface DashboardResponse {
+  summary: {
+    down: number;
+    incidents: number;
+    lastCheckTime: null | string;
+    total: number;
+    up: number;
+    window: {
+      from: string;
+      label: string;
+      to: string;
+    };
+  };
+  targets: TargetStatus[];
+}
+
 export interface HealthResponse {
   ok: true;
   service: "dial-server";
@@ -45,19 +67,38 @@ export interface RecentSample {
 
 export type RuntimeMode = "development" | "production" | "test";
 
-export interface SummaryResponse {
-  down: number;
-  lastCheckTime: null | string;
-  total: number;
-  up: number;
+export interface TargetMetricsResponse {
+  stats: {
+    availability: number;
+    avgDurationMs: null | number;
+    currentStreak: CurrentStreak | null;
+    downChecks: number;
+    incidentCount: number;
+    longestOutage: null | number;
+    mttr: null | number;
+    p95DurationMs: null | number;
+    p99DurationMs: null | number;
+    totalChecks: number;
+    upChecks: number;
+  };
+  targetId: number;
+  trend: TrendPoint[];
+  window: {
+    bucket: "1h";
+    from: string;
+    to: string;
+  };
 }
 
 export interface TargetStats {
   availability: number;
+  downChecks: number;
   totalChecks: number;
+  upChecks: number;
 }
 
 export interface TargetStatus {
+  currentStreak: CurrentStreak | null;
   group: string;
   id: number;
   interval: string;
@@ -72,6 +113,10 @@ export interface TargetStatus {
 export interface TrendPoint {
   availability: number;
   avgDurationMs: null | number;
-  hour: string;
+  bucketStart: string;
+  downChecks: number;
+  maxDurationMs: null | number;
+  minDurationMs: null | number;
   totalChecks: number;
+  upChecks: number;
 }