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/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) ───────────────────────────────────