Instrument API timing and reduce detail fallbacks

frontend/app/api/city/[name]/detail/route.ts

@@ -9,6 +9,10 @@ import {
 } from "@/lib/api-proxy";
 import { buildCachedJsonResponse } from "@/lib/http-cache";
 import { buildCityDetailProxyCachePolicy } from "@/lib/proxy-cache-policy";
+import {
+  createProxyTimer,
+  finishProxyTimedResponse,
+} from "@/lib/proxy-timing";
 
 const API_BASE = process.env.POLYWEATHER_API_BASE_URL;
 
@@ -34,15 +38,16 @@ export async function GET(
   req: NextRequest,
   context: { params: Promise<{ name: string }> },
 ) {
+  const timer = createProxyTimer(req, "city_detail");
   if (!API_BASE) {
     const response = NextResponse.json(
       { error: "POLYWEATHER_API_BASE_URL is not configured" },
       { status: 500 },
     );
-    return response;
+    return finishProxyTimedResponse(response, timer, "missing_api_base");
   }
 
-  const { name } = await context.params;
+  const { name } = await timer.measure("route_params", () => context.params);
   const forceRefresh = req.nextUrl.searchParams.get("force_refresh") ?? "false";
   const cachePolicy = buildCityDetailProxyCachePolicy(forceRefresh, 15);
   const depth = req.nextUrl.searchParams.get("depth");
@@ -67,31 +72,48 @@ export async function GET(
   const url = `${API_BASE}/api/city/${encodeURIComponent(name)}/detail?${searchParams.toString()}`;
 
   try {
-    const auth = await buildBackendRequestHeaders(req, {
-      includeSupabaseIdentity: false,
-    });
-    const res = await fetch(url, {
-      headers: auth.headers,
-      ...(cachePolicy.fetchMode === "no-store"
-        ? { cache: "no-store" as const }
-        : { next: { revalidate: cachePolicy.revalidateSeconds ?? 15 } }),
-    });
+    const auth = await timer.measure("auth_headers", () =>
+      buildBackendRequestHeaders(req, {
+        includeSupabaseIdentity: false,
+      }),
+    );
+    const res = await timer.measure("backend_fetch", () =>
+      fetch(url, {
+        headers: auth.headers,
+        ...(cachePolicy.fetchMode === "no-store"
+          ? { cache: "no-store" as const }
+          : { next: { revalidate: cachePolicy.revalidateSeconds ?? 15 } }),
+      }),
+    );
+    const backendServerTiming = res.headers.get("server-timing") || "";
     if (!res.ok) {
-      const raw = await res.text();
+      const raw = await timer.measure("backend_read", () => res.text());
       const response = buildUpstreamErrorResponse(res.status, raw);
-      return applyAuthResponseCookies(response, auth.response);
+      return finishProxyTimedResponse(
+        applyAuthResponseCookies(response, auth.response),
+        timer,
+        `upstream_${res.status}`,
+        { backendServerTiming },
+      );
     }
-    const data = normalizeCityDetailPayload(await res.json());
+    const data = normalizeCityDetailPayload(
+      await timer.measure("backend_read", () => res.json()),
+    );
     const response = buildCachedJsonResponse(
       req,
       data,
       cachePolicy.responseCacheControl,
     );
-    return applyAuthResponseCookies(response, auth.response);
+    return finishProxyTimedResponse(
+      applyAuthResponseCookies(response, auth.response),
+      timer,
+      "ok",
+      { backendServerTiming },
+    );
   } catch (error) {
     const response = buildProxyExceptionResponse(error, {
       publicMessage: "Failed to fetch city detail aggregate",
     });
-    return response;
+    return finishProxyTimedResponse(response, timer, "exception");
   }
 }