fix(organizations): extend global-admin bypass to updateRole + centralize isGlobalAdmin helper

[PierreBrisorgueil]

Problem

Follow-up to #3505. Two related improvements surfaced during review that are intentionally out of scope for that PR:

1. modules/organizations/controllers/organizations.membership.controller.js#updateRole still requires the actor to be an org owner (req.membership.role === OWNER). Global platform admins hitting this handler (even when they are not a member of the target org) will get a 403. For consistency with the remove-member / delete-org bypass, admins should be able to change member roles too — notably to transfer ownership during moderation.

2. The pattern Array.isArray(req.user?.roles) && req.user.roles.includes('admin') is now repeated in organizations.controller.js, organizations.membership.controller.js, organizations.middleware.js, and organizations.policy.js. This should be centralized (e.g. req.isGlobalAdmin populated once in an auth middleware, or a small helper in lib/helpers/auth.js).

Expected behaviour

• Global admin can call PUT /api/organizations/:orgId/memberships/:memberId with { role: 'owner' | 'admin' | 'member' } and succeed even when not a member of the target org.
• A single source of truth for isGlobalAdmin(req) used consistently across the codebase.

Scope note

Item 1 must preserve the belt-and-suspenders block against CASL shipping update Membership by mistake for non-owner org roles — the fix is to allow only global admins + owners, not to loosen CASL overall.