fix(tree): address realtime-tree-server review findings

- make addTreeNode receivers idempotent (invalidateOnCreatePage guard +
  buildTree dedup) so the author's self-echo no longer duplicates the node
- broadcast realtime tree updates for bulk copy/duplicate and import via a
  root refetch: PAGE_CREATED now carries spaceId and the WS listener falls
  back to refetchRootTreeNodeEvent when no per-node snapshot is present
- remove the now-dead client-relay inbound path (isTreeEvent/handleTreeEvent)
  that remained a stale-restriction-cache attack surface
- honest string|null cast for a root move's parent id
- add tests: buildTree dedup; onPageCreated per-node vs refetch branching

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/server/src/ws/ws.utils.ts

@@ -8,11 +8,3 @@ export function getSpaceRoomName(spaceId: string): string {
 export function getUserRoomName(userId: string): string {
   return `user-${userId}`;
 }
-
-export const TREE_EVENTS = new Set([
-  'updateOne',
-  'addTreeNode',
-  'moveTreeNode',
-  'deleteTreeNode',
-  'refetchRootTreeNodeEvent',
-]);