In-Memory DB with per-key user lock: setByUser/deleteByUser/lock/unlock

Design a simplified in‑memory key‑value database that stores records and supports fine‑grained locking. Each stored key maps to a single record and may be locked by a specific user. The locking rules guarantee that only the lock owner may alter the locked entry. Implement the following four operations. Their exact signatures and return semantics are defined by the original assessment; the behavior described below captures the mandatory semantics. setByUser(user, key, …) –…