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/routes/federation.ts

@@ -1002,16 +1002,39 @@ function processCreateEvent(
   }
   const authorUser = authorEntry.localUser;
 
-  // Compute federated ID from participants' home user IDs and find/create channel
+  // Resolve local DM channel: group DMs carry a federatedId and the channel
+  // must already exist (bootstrapped by a prior member_add event); 1-on-1 DMs
+  // are computed from the pair of home user IDs and created on demand.
+  let localDmChannelId: string;
+
+  if (event.federatedId) {
+    // Group DM: look up by federated_id (channel must already exist from member_add bootstrap)
+    const channel = db
+      .select()
+      .from(schema.dmChannels)
+      .where(and(
+        eq(schema.dmChannels.federatedId, event.federatedId),
+        isNull(schema.dmChannels.deletedAt),
+      ))
+      .get();
+
+    if (!channel) {
+      rejected.push({ messageId: event.messageId, reason: 'channel_not_found' });
+      return;
+    }
+    localDmChannelId = channel.id;
+  } else {
+    // 1-on-1 DM: compute federated_id from pair and find/create channel
     const federatedId = computeFederatedId(
       resolvedParticipants[0]!.homeUserId,
       resolvedParticipants[1]!.homeUserId,
     );
-  const localDmChannelId = findOrCreateDmChannel(
+    localDmChannelId = findOrCreateDmChannel(
       federatedId,
       [resolvedParticipants[0]!.localUser.id, resolvedParticipants[1]!.localUser.id],
       db,
     );
+  }
 
   // Insert the message
   const localMessageId = generateSnowflake();

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,