A SqlFileSystem over a hosted Turso database. Vercel functions are
stateless, so persistence lives in Turso — files written in one request survive for
the next. Same backend story as Cloudflare, where the handle is a Durable Object's
ctx.storage.sql.
curl -X POST "https://fs-demo-vercel.vercel.app/api/write" -H 'content-type: application/json' -d '{"path":"/kb/hours.md","content":"Open 9-5"}'
curl "https://fs-demo-vercel.vercel.app/api/read?path=/kb/hours.md" # persisted in Turso
curl "https://fs-demo-vercel.vercel.app/api/ls?path=/kb"