feat: outbound S2S read state relay

Queue read_state_update events when users ack DM messages on
channels with a federatedId. Translates local message IDs to
federation coordinates using sourceInstance/sourceMessageId.

packages/server/src/ws/events.ts

@@ -11,7 +11,7 @@ import { ACTIVITY_LIMITS } from '@backspace/shared/src/activities.js';
 import { sanitizeUser } from '../utils/sanitize.js';
 import { deleteAttachmentFiles } from '../utils/fileCleanup.js';
 import { resolveEmbeds, reResolveEmbeds, embedRowToEmbed } from '../utils/embedResolver.js';
-import { appendMutationLog, queueOutboxEvent, queueDmRelay, getGroupDmTargetOrigins, sendCallRelay, computeFederatedId, sendTypingRelay } from '../utils/federationOutbox.js';
+import { appendMutationLog, queueOutboxEvent, queueDmRelay, getGroupDmTargetOrigins, sendCallRelay, computeFederatedId, sendTypingRelay, queueReadStateRelay } from '../utils/federationOutbox.js';
 import { getOurOrigin } from '../utils/federationAuth.js';
 import { generateFederatedCallToken } from '../routes/livekit.js';
 import { config } from '../config.js';
@@ -1320,6 +1320,9 @@ function handleChannelAck(event: Record<string, unknown>, userId: string, isFede
     channelId,
     messageId,
   });
+
+  // Relay read state to federated peers for cross-instance sync
+  queueReadStateRelay(channelId, messageId, userId);
 }
 
 function handleMarkUnread(event: Record<string, unknown>, userId: string, isFederated: boolean): void {