feat: 结构化 observation 替代 statusDetail，API 层动态构造 detail

- CheckResult: statusDetail -> observation (持久化) + detail (API 动态派生)
- 存储: status_detail 列 -> observation TEXT (JSON)
- CheckerDefinition: 新增 buildDetail(observation) 方法
- 各 checker 返回结构化 observation，API 层通过 registry 调用 buildDetail
- HTTP: bodyPreview 在 status/header 失败时也提前采集
- UDP: observation 包含 durationMs，未响应归为 error failure
- CMD: 超时/输出超限时保留已收集 observation
- TCP: connectTimeMs 仅含连接建立耗时，不含 banner 等待
- 新增 buildDetail 单测和 mapCheckResult 覆盖测试
- 同步 openspec 主规范，归档 checker-observation 变更

tests/server/helpers.test.ts

@@ -1,6 +1,8 @@
 import { describe, expect, test } from "bun:test";
 
-import { createApiError, createHeaders, formatDuration, jsonResponse } from "../../src/server/helpers";
+import type { StoredCheckResult } from "../../src/server/checker/types";
+
+import { createApiError, createHeaders, formatDuration, jsonResponse, mapCheckResult } from "../../src/server/helpers";
 
 describe("createApiError", () => {
   test("创建错误响应对象", () => {
@@ -107,3 +109,42 @@ describe("formatDuration", () => {
     expect(formatDuration(61123)).toBe("61123ms");
   });
 });
+
+function makeRow(overrides: Partial<StoredCheckResult> = {}): StoredCheckResult {
+  return {
+    duration_ms: 12,
+    failure: null,
+    id: 1,
+    matched: 1,
+    observation: null,
+    target_id: "target-1",
+    timestamp: "2025-01-01T00:00:00.000Z",
+    ...overrides,
+  };
+}
+
+describe("mapCheckResult", () => {
+  test("反序列化 observation 并构造 detail", () => {
+    const result = mapCheckResult(makeRow({ observation: JSON.stringify({ statusCode: 200 }) }), "http");
+    expect(result.detail).toBe("HTTP 200");
+    expect(result.observation).toEqual({ statusCode: 200 });
+  });
+
+  test("null observation 返回 null detail", () => {
+    const result = mapCheckResult(makeRow(), "http");
+    expect(result.detail).toBeNull();
+    expect(result.observation).toBeNull();
+  });
+
+  test("未知 type 不影响响应序列化", () => {
+    const result = mapCheckResult(makeRow({ observation: JSON.stringify({ statusCode: 200 }) }), "unknown");
+    expect(result.detail).toBeNull();
+    expect(result.observation).toEqual({ statusCode: 200 });
+  });
+
+  test("损坏 observation JSON 返回 null observation", () => {
+    const result = mapCheckResult(makeRow({ observation: "{invalid json" }), "http");
+    expect(result.detail).toBeNull();
+    expect(result.observation).toBeNull();
+  });
+});