补全 scan/terminal/overview Next.js API route handler，修复生产 404

MarketOverviewBanner 改为使用 fetchBackendApi() 统一调用模式，
同时创建缺失的 app/api/scan/terminal/overview/route.ts 代理到后端。
其他 scan/terminal 子路由 (ai, ai-city, stream) 已有对应 handler。

frontend/app/api/scan/terminal/overview/route.ts

@@ -0,0 +1,75 @@
+import { NextRequest, NextResponse } from "next/server";
+import {
+  applyAuthResponseCookies,
+  buildBackendRequestHeaders,
+} from "@/lib/backend-auth";
+import {
+  buildProxyExceptionResponse,
+  buildUpstreamErrorResponse,
+} from "@/lib/api-proxy";
+
+const API_BASE = process.env.POLYWEATHER_API_BASE_URL;
+const OVERVIEW_PROXY_TIMEOUT_MS = Math.max(
+  35_000,
+  Number(process.env.POLYWEATHER_SCAN_OVERVIEW_PROXY_TIMEOUT_MS || "45000") || 45_000,
+);
+
+export const dynamic = "force-dynamic";
+export const maxDuration = 60;
+
+export async function POST(req: NextRequest) {
+  if (!API_BASE) {
+    return NextResponse.json(
+      { error: "POLYWEATHER_API_BASE_URL is not configured" },
+      { status: 500 },
+    );
+  }
+
+  let body: unknown = {};
+  try {
+    body = await req.json();
+  } catch {
+    body = {};
+  }
+
+  let auth: Awaited<ReturnType<typeof buildBackendRequestHeaders>> | null = null;
+  const controller = new AbortController();
+  const timeoutId = setTimeout(() => controller.abort(), OVERVIEW_PROXY_TIMEOUT_MS);
+
+  try {
+    auth = await buildBackendRequestHeaders(req);
+    const headers = new Headers(auth.headers);
+    headers.set("Content-Type", "application/json");
+    headers.set("Accept", "application/json");
+    const res = await fetch(`${API_BASE}/api/scan/terminal/overview`, {
+      method: "POST",
+      headers,
+      cache: "no-store",
+      signal: controller.signal,
+      body: JSON.stringify(body || {}),
+    });
+    if (!res.ok) {
+      const raw = await res.text();
+      const response = buildUpstreamErrorResponse(res.status, raw);
+      return applyAuthResponseCookies(response, auth.response);
+    }
+    const data = await res.json();
+    const response = NextResponse.json(data, {
+      headers: {
+        "Cache-Control": "no-store",
+      },
+    });
+    return applyAuthResponseCookies(response, auth.response);
+  } catch (error) {
+    const timedOut = controller.signal.aborted;
+    const response = buildProxyExceptionResponse(error, {
+      publicMessage: timedOut
+        ? "Market overview request timed out"
+        : "Failed to fetch market overview",
+      status: timedOut ? 504 : 500,
+    });
+    return auth ? applyAuthResponseCookies(response, auth.response) : response;
+  } finally {
+    clearTimeout(timeoutId);
+  }
+}