Cleanup user table

There are unnecessary columns in the users table.

We should **delete**:
- globus_identity
- namespace

We could **optionally delete**:
- deleted -- this was added for the initial GUI but is no longer used.

Start by locating the users table definition and its migration history, then search the repository for globus_identity, namespace, and deleted. Confirm which columns are still referenced and inspect the database-related tests or migration checks. Done means the unused columns are removed safely, with deleted handled according to the project’s decision.

Written by the indexing model from the issue text.