feat: Add roles for users within backend

[sh-ran]

Contributor checklist

• This pull request is on a separate branch and not the main branch
• I have run the tests for the backend and frontend depending on what's needed for my changes as described in the testing section of the contributing guide

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Description

This PR adds roles for users within the backend. These roles will be used to check/obtain permissions within the backend for GET, POST, PUT and DELETE api calls done by users.

Related issue

• closes Add user roles for organizations in backend and frontend #1973

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[andrewtavis]

note: We're switching this to a draft as @sh-ran still has some work to do to clean the implementation up :)

[aasimsyed]

Just flagging one thing early since it affects everything downstream: role is added alongside the existing is_owner/is_admin/is_comms booleans, and CI is currently failing because the new unique_org_membership/unique_group_membership constraints conflict with existing tests that create multiple membership rows per (org/group, user).

Does role fully replace the booleans (needs a migration/backfill), or coexist for a while?

[andrewtavis]

Thanks for noting this, @aasimsyed! Depends on the speed here. Might replace if we can get this done in say the next two weeks to a month.