Attributed to the admin doing the adding rather than a new system/bot
sender concept, since every message today requires a real user_id and
the admin is already a real, in-scope user for the request. Broadcasts
room_added before the welcome message itself, so the new member's
client learns the room exists before it sees an unread update for it.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>