🚀 Deployment: Final fix for backend 500 error (resilient startup + folder entrypoint)

🐛 Fix: Resolve backend 500 on Vercel by implementing FastAPI and syncing dependencies

🔧 Deployment: Explicitly set framework slugs in vercel.json

🔧 Fix: Added missing entrypoint for frontend service in vercel.json

🔧 Deployment: Add vercel.json for experimental multi-service support