refactor: replace user_id with workspace_id in Brain class

[StanGirard]

Description

Please include a summary of the changes and the related issue. Please also include relevant motivation and context.

Checklist before requesting a review

Please delete options that are not relevant.

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented hard-to-understand areas
• I have ideally added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

Screenshots (if appropriate):