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-columns.test.ts

@@ -19,13 +19,14 @@ function getColumn(columns: Array<PrimaryTableCol<TargetStatus>>, colKey: string
 
 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,
@@ -33,7 +34,7 @@ function makeTarget(overrides: Partial<TargetStatus> = {}): TargetStatus {
 }
 
 describe("createTargetTableColumns", () => {
-  test("生成 7 个目标表格列", () => {
+  test("生成 8 个目标表格列", () => {
     const columns = createTargetTableColumns(["http", "cmd"]);
 
     expect(columns.map((column) => column.colKey)).toEqual([
@@ -42,6 +43,7 @@ describe("createTargetTableColumns", () => {
       "type",
       "stats.availability",
       "recentSamples",
+      "currentStreak",
       "latestCheck.durationMs",
       "interval",
     ]);
@@ -81,4 +83,19 @@ describe("createTargetTableColumns", () => {
 
     expect(element.props.children).toBe("tcp");
   });
+
+  test("连续状态列渲染 capped 标记", () => {
+    const streakColumn = getColumn(createTargetTableColumns(["http"]), "currentStreak");
+    const renderCell = streakColumn.cell as (params: PrimaryTableCellParams<TargetStatus>) => {
+      props: { children: unknown[] };
+    };
+    const element = renderCell({
+      col: streakColumn,
+      colIndex: 5,
+      row: makeTarget({ currentStreak: { capped: true, count: 30, up: false } }),
+      rowIndex: 0,
+    });
+
+    expect(element.props.children.join("")).toBe("▼ 30+");
+  });
 });