juanjo finishes ripping Supabase out of Mike

This is the closing move in a larger swap. juanjo's branch replaces Supabase - the hosted database-and-auth service Mike originally leaned on - with a fully local stack: Postgres in a container, Auth.js for sign-in, and Drizzle managing the schema. Object storage moves the same way, with MinIO standing in locally for what S3 or Cloudflare R2 would handle in production.

This particular slice is the cleanup: the last Supabase package is uninstalled, the leftover environment variables are scrubbed, and the README's setup instructions are rewritten so a new developer follows the new path. The old database migration file gets a deprecated header rather than a deletion. The practical result is that the project now boots from a single command with no external accounts required for first-run development.