feat: log BACKEND_URL at frontend startup and include target URL in proxy errors

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
Agent-Logs-Url: https://github.com/christianlouis/pop_puller_to_gmail/sessions/131238a2-437c-4a85-b98b-fcb3d80e69e0

frontend/src/app/api/v1/[...path]/route.ts

@@ -29,7 +29,7 @@ async function handler(request: NextRequest, context: RouteContext) {
       headers: upstream.headers,
     });
   } catch (error) {
-    console.error("Proxy error forwarding to backend:", error);
+    console.error(`Proxy error forwarding to backend (target: ${targetUrl}):`, error);
     return NextResponse.json({ detail: "Backend unavailable" }, { status: 502 });
   }
 }

frontend/src/instrumentation.ts

@@ -0,0 +1,4 @@
+export async function register() {
+  const backendUrl = process.env.BACKEND_URL ?? "http://localhost:8000";
+  console.log(`[proxy] BACKEND_URL = ${backendUrl}`);
+}