chore(deploy): Set up Neon database and restore hosted PostgreSQL

[mathemage]

Why

The current hosted login incident was recovered with an emergency SQLite-based deploy, but the long-term deployment path still needs a durable PostgreSQL database and a stable DATABASE_URL in Vercel.

Neon is the preferred provider for this project because it keeps the database independent from Vercel while fitting the app's preview and production workflow well.

What changed

• create a Neon account or organization for the project
• provision the production PostgreSQL database in Neon
• provision a safe preview database or branch strategy for Vercel preview deployments
• add DATABASE_URL to Vercel for Production and Preview
• migrate or restore the current hosted data into Neon
• switch the app back from the emergency SQLite recovery path to the normal PostgreSQL-backed deployment flow
• document the final Neon + Vercel setup steps in the README if needed

Acceptance criteria

• production uses a Neon PostgreSQL database via DATABASE_URL
• preview deployments also have a valid PostgreSQL DATABASE_URL strategy
• https://deep-workflow.vercel.app/health/ready/ returns database ok on the normal Git-based deployment path
• login and core daily-sheet flows work against Neon-backed production
• the emergency SQLite-only hosted fallback is no longer required for normal production deploys

Out of scope

• changing the core product workflow
• adding analytics or unrelated infrastructure services