Add cities static fallback

frontend/app/api/cities/route.ts

@@ -1,21 +1,59 @@
-import { NextRequest, NextResponse } from "next/server";
+import { NextRequest } from "next/server";
 import { proxyBackendJsonGet } from "@/lib/api-proxy";
+import { buildCachedJsonResponse } from "@/lib/http-cache";
+import { STATIC_CITY_LIST } from "@/lib/static-cities";
 
 const API_BASE = process.env.POLYWEATHER_API_BASE_URL;
+const CITIES_CACHE_CONTROL = "public, max-age=0, s-maxage=60, stale-while-revalidate=300";
+const STATIC_CITIES_CACHE_CONTROL =
+  "public, max-age=0, s-maxage=300, stale-while-revalidate=3600";
+const CITIES_BACKEND_TIMEOUT_MS = Number(
+  process.env.POLYWEATHER_CITIES_BACKEND_TIMEOUT_MS || 2500,
+);
+
+function staticCitiesFallback(req: NextRequest, reason: string) {
+  const response = buildCachedJsonResponse(
+    req,
+    {
+      cities: STATIC_CITY_LIST,
+      source: "static_fallback",
+      stale: true,
+    },
+    STATIC_CITIES_CACHE_CONTROL,
+  );
+  response.headers.set("x-polyweather-cities-source", "static-fallback");
+  response.headers.set("x-polyweather-cities-fallback-reason", reason);
+  return response;
+}
 
 export async function GET(req: NextRequest) {
   if (!API_BASE) {
-    const response = NextResponse.json(
-      { error: "POLYWEATHER_API_BASE_URL is not configured" },
-      { status: 500 },
-    );
-    return response;
+    return staticCitiesFallback(req, "missing-api-base");
   }
 
-  return proxyBackendJsonGet(req, {
-    cacheControl: "public, max-age=0, s-maxage=60, stale-while-revalidate=300",
-    publicMessage: "Failed to fetch cities",
-    revalidateSeconds: 60,
-    url: `${API_BASE}/api/cities`,
-  });
+  const abortController = new AbortController();
+  const timeout = setTimeout(() => abortController.abort(), CITIES_BACKEND_TIMEOUT_MS);
+
+  try {
+    const response = await proxyBackendJsonGet(req, {
+      cacheControl: CITIES_CACHE_CONTROL,
+      publicMessage: "Failed to fetch cities",
+      revalidateSeconds: 60,
+      signal: abortController.signal,
+      statusOnException: 504,
+      timeoutPublicMessage: "Cities backend timed out",
+      url: `${API_BASE}/api/cities`,
+    });
+
+    if (response.status >= 500) {
+      return staticCitiesFallback(
+        req,
+        response.status === 504 ? "backend-timeout" : "backend-error",
+      );
+    }
+
+    return response;
+  } finally {
+    clearTimeout(timeout);
+  }
 }