fix(federation): fix reaction relay using wrong ID for message lookup

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.

packages/shared/src/types.ts

@@ -744,6 +744,7 @@ export interface FederationRelayEvent {
 }
 
 export interface FederationRelayReaction {
+  messageId?: string;
   userId: string;
   homeUserId: string;
   emoji: string;