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

tests/web/constants/target-table-sorters.test.ts

@@ -11,13 +11,14 @@ import {
 
 function makeTarget(overrides: Partial<TargetStatus> = {}): TargetStatus {
   return {
+    currentStreak: null,
     group: "default",
     id: 1,
     interval: "5s",
     latestCheck: null,
     name: "test",
     recentSamples: [],
-    stats: { availability: 100, totalChecks: 0 },
+    stats: { availability: 100, downChecks: 0, totalChecks: 0, upChecks: 0 },
     target: "https://example.com",
     type: "http",
     ...overrides,
@@ -57,20 +58,20 @@ describe("statusSorter", () => {
 
 describe("availabilitySorter", () => {
   test("低可用率排前面", () => {
-    const low = makeTarget({ stats: { availability: 95, totalChecks: 100 } });
-    const high = makeTarget({ stats: { availability: 99.9, totalChecks: 100 } });
+    const low = makeTarget({ stats: { availability: 95, downChecks: 5, totalChecks: 100, upChecks: 95 } });
+    const high = makeTarget({ stats: { availability: 99.9, downChecks: 1, totalChecks: 100, upChecks: 99 } });
     expect(availabilitySorter(low, high)).toBeLessThan(0);
   });
 
   test("相同可用率返回 0", () => {
-    const a = makeTarget({ stats: { availability: 99.9, totalChecks: 100 } });
-    const b = makeTarget({ stats: { availability: 99.9, totalChecks: 50 } });
+    const a = makeTarget({ stats: { availability: 99.9, downChecks: 1, totalChecks: 100, upChecks: 99 } });
+    const b = makeTarget({ stats: { availability: 99.9, downChecks: 1, totalChecks: 50, upChecks: 49 } });
     expect(availabilitySorter(a, b)).toBe(0);
   });
 
   test("无 stats 按 0 处理", () => {
     const noStats = makeTarget({ stats: undefined as unknown as TargetStatus["stats"] });
-    const high = makeTarget({ stats: { availability: 99.9, totalChecks: 100 } });
+    const high = makeTarget({ stats: { availability: 99.9, downChecks: 1, totalChecks: 100, upChecks: 99 } });
     expect(availabilitySorter(noStats, high)).toBeLessThan(0);
   });
 });