Root cause: publish_reaction_event sends a RoomMessageEvent (not
reaction_added) with reactions field set. The onRoomMessage handler
previously returned prev immediately when a duplicate ID was found,
skipping the reaction update entirely.
Fix:
- Add reactions field to RoomMessagePayload so TypeScript knows it's there
- When a duplicate message ID is found AND payload carries reactions,
update the existing message's reactions field instead of ignoring
- ReactionItem and ReactionGroup have identical shapes, so assignment works
