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

web/routers/scan.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 from fastapi import APIRouter, Request
+from fastapi.responses import JSONResponse
 
 from web.services.scan_api import (
     get_scan_terminal_ai_payload,
@@ -30,7 +31,7 @@ async def scan_terminal(
     trading_region: str = "",
     timezone_offset_seconds: int | None = None,
 ):
-    return await get_scan_terminal_payload(
+    payload = await get_scan_terminal_payload(
         request,
         scan_mode=scan_mode,
         min_price=min_price,
@@ -45,6 +46,12 @@ async def scan_terminal(
         region=region or trading_region or None,
         timezone_offset_seconds=timezone_offset_seconds,
     )
+    return JSONResponse(
+        content=payload,
+        headers={
+            "Cache-Control": "public, s-maxage=30, stale-while-revalidate=120",
+        },
+    )
 
 
 @router.post("/api/scan/terminal/ai")