Scoped to direct messages only, and deliberately narrower than the
existing push/desktop "offline" (not connected to this room's channel
right now, which fires on every message) -- email uses GlobalPresence
instead (no open connection anywhere, or appear_offline), since the
other participant could easily just be active in a different room.
Debounced to the first unread message in the conversation rather than
firing on every message in a burst, resetting once they mark it read.
Reuses the existing SMTP/send_email infrastructure from the invite
feature, so it silently no-ops if SMTP isn't configured, same as
everywhere else that already uses it.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>