fix(image): add response-header timeout and idempotent retry

Give the image client a configurable ResponseHeaderTimeout (default 4m,
capped at the overall timeout) so a stalled upstream fails fast, and
retry such timeouts when an idempotency key is set so a slow-but-safe
request can recover instead of terminating. Raise the Next.js API proxy
maxDuration to 1200s to accommodate long generations.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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

@@ -1,7 +1,7 @@
 import type { NextRequest } from "next/server";
 
 export const dynamic = "force-dynamic";
-export const maxDuration = 120;
+export const maxDuration = 1200;
 export const runtime = "nodejs";
 
 const apiBase = () => process.env.NEXT_PUBLIC_API_PROXY ?? "http://localhost:8888";