Have users have two ID

**Is your feature request related to a problem? Please describe.**
I want the users to have to IDs. One could be the external one that should be an UUID or come from an external provider, as the sub in OpenID Connect, and then have an autoincrementative int as the internal id for PK-FK relationships between tables.

The issue does not identify files, tests, or entry points in the Python REST API. First clarify the identifier model, migration and compatibility requirements, then trace the existing user and relationship models; completion criteria should explicitly define both identifiers and their use in API responses and database relationships.

Written by the indexing model from the issue text.