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/middleware.ts

@@ -0,0 +1,58 @@
+import type { RuntimeMode } from "../shared/api";
+import { allowsGetHead, createApiError, jsonResponse, methodNotAllowedResponse } from "./helpers";
+
+export function guardGetHead(method: string, mode: RuntimeMode): Response | null {
+  if (!allowsGetHead(method)) {
+    return methodNotAllowedResponse(["GET", "HEAD"], mode);
+  }
+  return null;
+}
+
+export function validateTargetId(idStr: string, mode: RuntimeMode): { id: number } | Response {
+  const id = Number(idStr);
+  if (!Number.isInteger(id) || id <= 0) {
+    return jsonResponse(createApiError("Invalid target ID", 400), { mode, status: 400 });
+  }
+  return { id };
+}
+
+export function validateTimeRange(
+  from: string | null,
+  to: string | null,
+  mode: RuntimeMode,
+): { from: string; to: string } | Response {
+  if (!from || !to) {
+    return jsonResponse(createApiError("from and to parameters are required", 400), { mode, status: 400 });
+  }
+
+  if (isNaN(new Date(from).getTime()) || isNaN(new Date(to).getTime())) {
+    return jsonResponse(createApiError("Invalid from or to parameter format", 400), { mode, status: 400 });
+  }
+
+  return { from, to };
+}
+
+export function validatePagination(
+  pageParam: string | null,
+  pageSizeParam: string | null,
+  mode: RuntimeMode,
+): { page: number; pageSize: number } | Response {
+  let page = 1;
+  let pageSize = 20;
+
+  if (pageParam !== null) {
+    page = Number(pageParam);
+    if (!Number.isInteger(page) || page <= 0) {
+      return jsonResponse(createApiError("Invalid page parameter", 400), { mode, status: 400 });
+    }
+  }
+
+  if (pageSizeParam !== null) {
+    pageSize = Number(pageSizeParam);
+    if (!Number.isInteger(pageSize) || pageSize <= 0) {
+      return jsonResponse(createApiError("Invalid pageSize parameter", 400), { mode, status: 400 });
+    }
+  }
+
+  return { page, pageSize };
+}