Reaction outbox events used reactionId (add) or a composite dedup key
(remove) as the event messageId. The receiver looked up dm_messages by
sourceMessageId = event.messageId, which never matched because it was
searching for a reactionId, not the actual message ID.
Fix: include the actual DM messageId in the reaction payload JSON.
The receiver now uses event.reaction.messageId for the lookup, with
fallback to event.messageId for backward compatibility.
Jannis Braun
a21243256e