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/app/api/analytics/events/route.ts

@@ -26,7 +26,9 @@ export async function POST(req: NextRequest) {
 
   try {
     const body = await req.json();
-    const auth = await buildBackendRequestHeaders(req);
+    const auth = await buildBackendRequestHeaders(req, {
+      includeSupabaseIdentity: false,
+    });
     const headers = new Headers(auth.headers);
     headers.set("Content-Type", "application/json");
     const res = await fetch(`${API_BASE}/api/analytics/events`, {