Email a DM's recipient when they're genuinely offline (#66)

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>

backend/app/ws/chat.py

@@ -230,7 +230,15 @@ async def chat_endpoint(websocket: WebSocket, db: AsyncSession = Depends(get_db)
                     # attributed sender may not actually be watching.
                     await mark_room_read(db, envelope.room_id, user.id)
                     await broadcast_new_message(
-                        db, broadcaster, presence, focus_presence, envelope.room_id, message, user
+                        db,
+                        broadcaster,
+                        presence,
+                        focus_presence,
+                        global_presence,
+                        str(websocket.base_url),
+                        envelope.room_id,
+                        message,
+                        user,
                     )
 
                 elif envelope.type == "edit":