ci(web): deploy merged web changes automatically

[ProdigyV21]

What changed

• deploy web/** changes automatically when they land on main
• build using the Netlify production context
• upload the generated Next.js function source so its catch-all route is preserved
• verify both the live HTTP response and exact build stamp after deployment
• serialize production deploys and cancel superseded runs

Validation

• npx yaml-lint .github/workflows/deploy-web.yml
• npx netlify-cli@25.6.0 build --context production
• verified the same static/function packaging against web.arvio.tv

The required Netlify token and web site ID are configured as encrypted repository secrets.