Step-by-step guide to building user authentication in an Astro application using Lucia Auth and Neon Serverless Postgres. Covers provisioning a Neon database, setting up the database schema with user and session tables, configuring Lucia Auth with a PostgreSQL adapter, building sign-up/sign-in/sign-out API routes and HTML pages, protecting routes from unauthenticated access, and deploying the finished app to Vercel.