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/static.ts

@@ -0,0 +1,54 @@
+import type { RuntimeMode } from "../shared/api";
+import { createHeaders } from "./helpers";
+import type { StaticAssets } from "./app";
+
+export function serveStaticAsset(pathname: string, staticAssets: StaticAssets, mode: RuntimeMode): Response {
+  if (pathname === "/") {
+    return htmlResponse(staticAssets.indexHtml, mode);
+  }
+
+  const asset = staticAssets.files[pathname];
+
+  if (asset) {
+    return new Response(asset, {
+      headers: createHeaders(mode, {
+        "Content-Type": contentTypeFor(pathname),
+        "Cache-Control": "public, max-age=31536000, immutable",
+      }),
+    });
+  }
+
+  if (pathname.startsWith("/assets/") || hasFileExtension(pathname)) {
+    return new Response("Not Found", {
+      status: 404,
+      headers: createHeaders(mode, { "Content-Type": "text/plain; charset=utf-8" }),
+    });
+  }
+
+  return htmlResponse(staticAssets.indexHtml, mode);
+}
+
+export function htmlResponse(indexHtml: Blob, mode: RuntimeMode): Response {
+  return new Response(indexHtml, {
+    headers: createHeaders(mode, {
+      "Content-Type": "text/html; charset=utf-8",
+      "Cache-Control": "no-cache",
+    }),
+  });
+}
+
+export function hasFileExtension(pathname: string): boolean {
+  return /\/[^/]+\.[^/]+$/.test(pathname);
+}
+
+export function contentTypeFor(pathname: string): string {
+  if (pathname.endsWith(".js") || pathname.endsWith(".mjs")) return "text/javascript; charset=utf-8";
+  if (pathname.endsWith(".css")) return "text/css; charset=utf-8";
+  if (pathname.endsWith(".svg")) return "image/svg+xml";
+  if (pathname.endsWith(".json")) return "application/json; charset=utf-8";
+  if (pathname.endsWith(".png")) return "image/png";
+  if (pathname.endsWith(".jpg") || pathname.endsWith(".jpeg")) return "image/jpeg";
+  if (pathname.endsWith(".ico")) return "image/x-icon";
+
+  return "application/octet-stream";
+}