Choose which admins are support agents

Imported from GitHub issue El-SaMa/oma#132 by @El-SaMa.

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User story

As an admin, I want to choose which admins are support agents, so that the ticket assignee list is the people who work tickets.

Background

The assignee dropdown is filled from staff-org memberships whose role is owner, admin, or support (listSupportAgents in apps/web/lib/support/agents.ts). Admin support settings have no control for this. A person can be an admin in practice and still be missing from the list if their membership role is something else.

Lived: the desk showed only Noora until Elsama's staff-org membership was updated in the database.

Acceptance criteria

• Admin, Support settings lists staff-org admins and owners.
• An admin can mark which of them are ticket assignees.
• The assignee dropdown on the desk and on a ticket shows only the marked people.
• Turning someone off does not delete their account and does not close tickets already assigned to them.
• A ticket that is already assigned to someone who is later turned off still shows that name.

Out of scope

• Inviting a new user.
• Changing their portal role.