feat: Add support for global wildcard scope assignation

[rodmgwgu]

Description

Closes: ...

Cover an existing gap on RBAC Authz Milestone 2: A superadmin user should be able to assign a role to a user for the whole platform.

This PR adds support for assigning roles to the global "*" scope. Only superusers or staff users are allowed to assign roles for this scope.

Deadline: Verawood

How to test

TODO

Other information

Co-authored by Claude Opus 4.6.

Merge checklist:

Check off if complete or not applicable:

• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets

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