Ensure thread-safe updates to org hierarchy

For the aforementioned organization hierarchy, design thread-safe add/remove operations for users and groups. Propose an API and concurrency strategy (e.g., reader–writer locks, fine-grained locking, lock ordering to avoid deadlocks, or immutable snapshots with copy-on-write). Explain how to guarantee linearizable or at least read-consistent queries while updates occur, and how to handle long-running reads. Discuss contention hot spots, performance trade-offs, failure recovery, and how to test concurrency correctness (stress tests, race detection).