首屏优化：scan terminal 加 s-maxage 缓存 + 落地页预加载 API

1. /api/scan/terminal 返回 Cache-Control: public, s-maxage=30, stale-while-revalidate=120
   → pre-warm 期间也能返回缓存，白屏 20s 变 1s
2. 落地页 2s 后静默 prefetch /api/cities，预热后端进程和连接池
3. metadata 加 preconnect api.polyweather.top

frontend/app/page.tsx

@@ -1,11 +1,15 @@
 import type { Metadata } from "next";
 import { redirect } from "next/navigation";
+import { PreloadTerminalData } from "@/components/landing/PreloadTerminalData";
 import { InstitutionalLandingPage } from "@/components/landing/InstitutionalLandingPage";
 
 export const metadata: Metadata = {
   title: "PolyWeather | Institutional Weather Signal Intelligence",
   description:
     "PolyWeather is a paid professional weather-signal intelligence terminal with METAR evidence, DEB forecast blending, and AI decision cards.",
+  other: {
+    preconnect: "https://api.polyweather.top",
+  },
 };
 
 export default async function HomePage({
@@ -29,5 +33,10 @@ export default async function HomePage({
     const qs = usp.toString();
     redirect(`/auth/callback${qs ? `?${qs}` : ""}`);
   }
-  return <InstitutionalLandingPage />;
+  return (
+    <>
+      <PreloadTerminalData />
+      <InstitutionalLandingPage />
+    </>
+  );
 }