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/routes/health.ts

@@ -0,0 +1,10 @@
+import type { RuntimeMode } from "../../shared/api";
+import { createHealthResponse, jsonResponse, allowsGetHead, methodNotAllowedResponse } from "../helpers";
+
+export function handleHealth(method: string, mode: RuntimeMode): Response {
+  if (!allowsGetHead(method)) {
+    return methodNotAllowedResponse(["GET", "HEAD"], mode);
+  }
+
+  return jsonResponse(createHealthResponse(), { method, mode });
+}