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/system/status/route.ts

@@ -7,6 +7,7 @@ import {
   buildProxyExceptionResponse,
   buildUpstreamErrorResponse,
 } from "@/lib/api-proxy";
+import { buildCachedJsonResponse } from "@/lib/http-cache";
 
 const API_BASE = process.env.POLYWEATHER_API_BASE_URL;
 
@@ -19,10 +20,12 @@ export async function GET(req: NextRequest) {
   }
 
   try {
-    const auth = await buildBackendRequestHeaders(req);
+    const auth = await buildBackendRequestHeaders(req, {
+      includeSupabaseIdentity: false,
+    });
     const res = await fetch(`${API_BASE}/api/system/status`, {
       headers: auth.headers,
-      cache: "no-store",
+      next: { revalidate: 30 },
     });
     if (!res.ok) {
       const raw = await res.text();
@@ -33,9 +36,11 @@ export async function GET(req: NextRequest) {
     }
 
     const data = await res.json();
-    const response = NextResponse.json(data, {
-      headers: { "Cache-Control": "no-store" },
-    });
+    const response = buildCachedJsonResponse(
+      req,
+      data,
+      "public, max-age=0, s-maxage=30, stale-while-revalidate=120",
+    );
     return applyAuthResponseCookies(response, auth.response);
   } catch (error) {
     return buildProxyExceptionResponse(error, {