fix(web): remove hardcoded maxDuration so Vercel dashboard setting takes effect

Code-level `export const maxDuration = 60` and vercel.json `functions`
block were overriding the dashboard's 300s setting, causing ~100 504
timeouts per day on /api/scene and /api/start. Removing them lets each
Vercel plan use its own default (60s Hobby, 300s Pro) without breaking
self-deployers.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/api/start/route.ts

@@ -4,7 +4,6 @@ import { NextResponse } from "next/server";
 import { loadEngineConfig } from "@/lib/config";
 
 export const runtime = "nodejs";
-export const maxDuration = 60;
 
 // Matches /api/vision and /api/parse-style-image — the user's resized 512px
 // webp is ~30-80 KB; this caps pathological direct-API payloads (which would