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/utils/time.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, test } from "bun:test";
 
-import { subtractHours } from "../../../src/web/utils/time";
+import { formatDurationUnit, formatRelativeTime, isOlderThan, subtractHours } from "../../../src/web/utils/time";
 
 describe("subtractHours", () => {
   test("正常扣减小时", () => {
@@ -27,3 +27,38 @@ describe("subtractHours", () => {
     expect(result.toISOString()).toBe("2025-01-15T12:00:00.000Z");
   });
 });
+
+describe("formatRelativeTime", () => {
+  const now = new Date("2025-01-01T00:02:00.000Z");
+
+  test("格式化秒和分钟", () => {
+    expect(formatRelativeTime("2025-01-01T00:01:45.000Z", now)).toBe("15秒前");
+    expect(formatRelativeTime("2025-01-01T00:00:00.000Z", now)).toBe("2分钟前");
+  });
+
+  test("无时间返回占位", () => {
+    expect(formatRelativeTime(null, now)).toBe("尚无检查数据");
+    expect(formatRelativeTime("invalid", now)).toBe("尚无检查数据");
+  });
+});
+
+describe("formatDurationUnit", () => {
+  test("按秒、分钟、小时动态格式化", () => {
+    expect(formatDurationUnit(1500)).toEqual({ suffix: "秒", value: 1.5 });
+    expect(formatDurationUnit(120000)).toEqual({ suffix: "分钟", value: 2 });
+    expect(formatDurationUnit(5400000)).toEqual({ suffix: "小时", value: 1.5 });
+  });
+
+  test("空时长返回占位", () => {
+    expect(formatDurationUnit(null)).toEqual({ suffix: "", value: 0 });
+  });
+});
+
+describe("isOlderThan", () => {
+  test("判断时间是否超过阈值", () => {
+    const now = new Date("2025-01-01T00:02:00.000Z");
+
+    expect(isOlderThan("2025-01-01T00:00:59.000Z", 60000, now)).toBe(true);
+    expect(isOlderThan("2025-01-01T00:01:30.000Z", 60000, now)).toBe(false);
+  });
+});