The #72 welcome message was attributed to whichever admin added the
member, since there was no system/bot sender concept -- every Message
row requires a real user_id. Adds a lazily-created "system" bot
account (reusing the existing is_bot infrastructure) and switches the
welcome message to post as it instead. Excluded from the People
directory and @mention autocomplete for free: the directory already
filters is_bot users, and mention suggestions are sourced from room
membership, which the system account is never added to.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>