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/integrations/import/services/file-import-task.service.ts

@@ -552,9 +552,13 @@ export class FileImportTaskService {
         }
 
         if (validPageIds.size > 0) {
+          // Carry the destination spaceId so the WS listener can trigger a root
+          // refetch for the imported subtree (no `pages` snapshot -> refetch
+          // fallback rather than per-node addTreeNode).
           this.eventEmitter.emit(EventName.PAGE_CREATED, {
             pageIds: Array.from(validPageIds),
             workspaceId: fileTask.workspaceId,
+            spaceId: fileTask.spaceId,
           });
         }