refactor: 全面优化后端代码质量与架构

- app.ts 单体路由拆分为 routes/ + helpers + middleware + static 独立模块
- 类型去重：CheckFailure/CheckResult 以 shared/api.ts 为唯一源头，收紧 phase 联合类型
- es-toolkit 替换：isPlainObject/isNil/isEmptyObject/isEqual/isError/Semaphore/groupBy
- Bun 内置 API：Object.fromEntries 替代手写 headersToRecord
- bun:sqlite 规范：prepare() → query() 利用内置缓存，避免 N+1 查询
- 新增 getLatestChecksMap/allGetTargetStats 批量查询方法
- 新增 backend-code-quality/api-route-separation/batch-data-queries 规范
- 补充 openspec/config.yaml 后端开发规范与 DEVELOPMENT.md 后端开发指引

src/server/checker/fetcher.ts

@@ -1,14 +1,7 @@
 import type { CheckResult, ResolvedHttpTarget } from "./types";
 import { checkHttpExpect } from "./expect/http";
 import { errorFailure } from "./expect/failure";
-
-function headersToRecord(headers: Headers): Record<string, string> {
-  const result: Record<string, string> = {};
-  headers.forEach((value, key) => {
-    result[key] = value;
-  });
-  return result;
-}
+import { isError } from "es-toolkit";
 
 export async function runHttpCheck(target: ResolvedHttpTarget): Promise<CheckResult> {
   const timestamp = new Date().toISOString();
@@ -27,7 +20,7 @@ export async function runHttpCheck(target: ResolvedHttpTarget): Promise<CheckRes
 
     const durationMs = Math.round(performance.now() - start);
     const statusCode = response.status;
-    const responseHeaders = headersToRecord(response.headers);
+    const responseHeaders = Object.fromEntries(response.headers);
 
     const hasBodyRules = !!(target.expect?.body && target.expect.body.length > 0);
 
@@ -93,7 +86,7 @@ export async function runHttpCheck(target: ResolvedHttpTarget): Promise<CheckRes
       failure: errorFailure(
         "status",
         "request",
-        isTimeout ? `请求超时 (${target.timeoutMs}ms)` : error instanceof Error ? error.message : String(error),
+        isTimeout ? `请求超时 (${target.timeoutMs}ms)` : isError(error) ? error.message : String(error),
       ),
     };
   }