fix: relay mark_unread to peers, fix docs and timestamp consistency

- Add queueReadStateRelay call in handleMarkUnread (skip '0' sentinel)
- Fix double Date.now() in queueReadStateRelay (use single const)
- Fix federation.md: read state relay uses outbox (not fire-and-forget),
  correct payload schema to match implementation

packages/server/src/utils/federationOutbox.ts

@@ -519,13 +519,14 @@ export function queueReadStateRelay(
     messageRef = { sourceInstance: ourOrigin, sourceMessageId: messageId };
   }
 
+  const now = Date.now();
   const payload: FederationRelayEvent = {
     eventType: 'read_state_update',
     dmChannelId: channelId,
-    messageId: `read_state:${userId}:${Date.now()}`,
+    messageId: `read_state:${userId}:${now}`,
     federatedId: channel.federatedId,
     encryptionVersion: 0,
-    timestamp: Date.now(),
+    timestamp: now,
     readState: {
       user: { homeUserId, homeInstance },
       messageRef,

packages/server/src/ws/events.ts

@@ -1384,6 +1384,12 @@ function handleMarkUnread(event: Record<string, unknown>, userId: string, isFede
     channelId,
     messageId,
   });
+
+  // Relay mark-unread to federated peers for cross-instance sync
+  // (skip the '0' sentinel — it deletes the read state and can't be mapped to a message)
+  if (messageId !== '0') {
+    queueReadStateRelay(channelId, messageId, userId);
+  }
 }
 
 // ─── DM Call Handlers (Unified Room API) ───────────────────────────────────