Allow admins to disable user accounts

[timothyfroehlich]

Context

Admins need the ability to disable user accounts, especially in cases where a password may be compromised. Currently there's no way to lock an account without deleting it.

Request

• Add an admin action to disable/suspend a user account
• Disabled accounts should not be able to log in
• Should be reversible (re-enable) unlike account deletion
• Key use case: compromised passwords where the user hasn't acted yet

Notes

• Account deletion already exists (see feat(settings): add account deletion with data anonymization #988), but that's a destructive, user-initiated action
• This is an admin-initiated, non-destructive safeguard