fix(vercel): add dual path resolution in api/index.py and main.py for serverless entrypoint

fix(vercel): add .vercelignore and function excludeFiles to prevent node_modules from inflating Python serverless zip

feat(monorepo): configure optimized fullstack Vercel monorepo deployment

fix(vercel): separate frontend build to bypass 500MB serverless bundle limit

feat: configure Vercel Python serverless function handler for FastAPI backend