Migrate hosting from Netlify to Vercel

Inizia ispezionando netlify.toml e le attuali impostazioni di build, pubblicazione, ambiente, header, redirect e dominio di @multei/web. Esamina il coordinamento con #415, quindi determina la configurazione di Vercel e le modifiche DNS necessarie. Il lavoro è completato quando la produzione e le anteprime delle PR funzionano su Vercel, i controlli smoke hanno esito positivo, gli artefatti Netlify vengono rimossi o contrassegnati come obsoleti e la documentazione del deploy viene aggiornata.

Context

The Multei! site (@multei/web) is hosted on Netlify (netlify.toml, gatsby build, publish public/, netlify-plugin-gatsby-cache). Production domain: https://multei.com.br.

Goal

Move deploy and hosting to Vercel, keeping the domain and production behavior, with preview deployments per PR.

Motivation

• Align hosting with the Vercel ecosystem (especially if we proceed with the Next.js migration in #415).
• Native preview URLs per branch/PR and simpler deploy DX.
• Unify env vars, domains, and observability in one place.
• Drop Netlify-only Gatsby plugins (netlify-plugin-gatsby-cache, netlify-cli).

Suggested scope

Preparation

• Inventory current Netlify config (build command, publish dir, env vars, redirects/headers, domain, DNS, forms/functions if any).
• Decide whether migration happens before or after Gatsby → Next.js (#415); ideal path is often Next.js + Vercel together, but Gatsby can run on Vercel temporarily.
• Create/link the Vercel project to multei/web.

Migration

• Configure Vercel build (Framework Preset: Gatsby now and/or Next.js after #415).
• Port security headers from netlify.toml (X-Frame-Options, X-XSS-Protection, etc.) to vercel.json / vercel.ts or framework headers.
• Port redirects/rewrites if any.
• Migrate environment variables (Production / Preview / Development).
• Configure multei.com.br (and www if applicable) on Vercel and update DNS.
• Validate preview deployments on PRs.

Cutover

• Smoke-test production (home, main routes, SEO/sitemap, assets).
• Remove or archive Netlify site/config after stabilizing (netlify.toml, netlify-cli, Netlify-only plugins).
• Update README/deploy docs.

Dependencies / related issues

• Related to #415 (Gatsby → Next.js). Prefer coordinating both to avoid two hosting cutovers.

Acceptance criteria

• Production served by Vercel at https://multei.com.br (HTTPS OK, correct redirects).
• Preview deployments work per PR.
• Env vars and headers equivalent to the current Netlify setup.
• Deploy docs updated; Netlify artifacts removed or marked obsolete after cutover.

Out of scope

• Redesign or product changes unrelated to hosting.
• Full framework migration (covered by #415), except the minimum needed to deploy on Vercel.