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>

vercel.json

@@ -1,11 +1,4 @@
 {
   "$schema": "https://openapi.vercel.sh/vercel.json",
-  "framework": "nextjs",
-  "functions": {
-    "app/api/start/route.ts":       { "maxDuration": 60 },
-    "app/api/scene/route.ts":       { "maxDuration": 60 },
-    "app/api/vision/route.ts":      { "maxDuration": 60 },
-    "app/api/insert-beat/route.ts": { "maxDuration": 60 },
-    "app/api/beat-audio/route.ts":  { "maxDuration": 30 }
-  }
+  "framework": "nextjs"
 }