优化 Vercel Fluid CPU：API加CDN缓存、缩小middleware范围、跳过公共API的Supabase身份转发

- 7条GET路由加 s-maxage + stale-while-revalidate，fetch 改为 next revalidate
- middleware matcher 从全匹配缩小到9条需auth的路径，移除 isStaticAsset
- 8条公共API路由加 includeSupabaseIdentity: false
- backend-auth getSession 为空时跳过 getUser
- subscription-help 加 I18nProvider，移除 force-dynamic → 静态预渲染
- next.config.mjs 加静态资源 immutable 缓存头

Tested: npx tsc --noEmit 通过，npm run build 通过

frontend/next.config.mjs

@@ -1,6 +1,17 @@
 /** @type {import('next').NextConfig} */
 const nextConfig = {
   reactStrictMode: true,
+  async headers() {
+    const cacheHeader = {
+      key: "Cache-Control",
+      value: "public, max-age=31536000, immutable",
+    };
+    const staticExts = ["jpg", "jpeg", "png", "gif", "ico", "svg", "webp", "avif", "woff2", "ttf", "eot", "css", "js"];
+    return staticExts.map((ext) => ({
+      source: `/:path(.+\\.${ext})`,
+      headers: [cacheHeader],
+    }));
+  },
 };
 
 export default nextConfig;