feat(federation): handle group DM messages via federatedId lookup in relay processor

In processCreateEvent, branch on event.federatedId: group DM messages now
look up the pre-bootstrapped local channel by federatedId instead of
computing a pair hash from two participants. In queueDmRelay, fetch the
channel's federatedId and ownerId and include federatedId in the outgoing
relay payload for group DMs so receiving instances can route correctly.

packages/server/src/utils/federationOutbox.ts

@@ -318,8 +318,17 @@ export function queueDmRelay(
 
   const targetOrigins = getGroupDmTargetOrigins(dmChannelId);
 
+  // Fetch channel to check if it's a group DM with a federatedId
+  const db = getDb();
+  const channel = db
+    .select({ federatedId: schema.dmChannels.federatedId, ownerId: schema.dmChannels.ownerId })
+    .from(schema.dmChannels)
+    .where(eq(schema.dmChannels.id, dmChannelId))
+    .get();
+
   appendMutationLog(message.id, dmChannelId, eventType);
   queueOutboxEvent(message.id, dmChannelId, eventType, JSON.stringify({
+    ...(channel?.federatedId && channel.ownerId ? { federatedId: channel.federatedId } : {}),
     message: {
       ...buildRelayPayload(message, message.user),
       attachments: attachments.length > 0 ? attachments : undefined,