Multi-App User Database

I need a a very flexible DB that we can add columns or tables easily to use it as a central DB for our current apps or in future. This will be a well-structured relational database that can act as the single source of truth for different apps. I like PGSQL DB

Please show me:
• An ER diagram that clearly documents every table, key, and relationship.
• Creation scripts (PostgreSQL or other ? whichever you recommend for cross-platform deployment) plus a concise data-dictionary.

This DB will allow user access based on certain parameters. Users can signup, login with full authentication.