ADMIN_USER_IDS (Telegram:AdminUserIds) takes a comma separated list; the older
ADMIN_USER_ID / Telegram:AdminUserId still works, also takes a list and is merged.
Empty entries and duplicates are ignored, non-numeric entries fail validation.
Impersonation is now per admin: one admin per target, an impersonating admin
can't be impersonated, and every admin's chat menu lists /impersonate.
While on, the admin's updates are routed as the target user's (allow-list,
chat, workspace, session, settings, jobs) and the target chat's output goes
to the admin's chat. The target's own updates are dropped with a notice that
an admin is working with their account. In memory; /impersonate 0 stops.
