Add _LOCK_RETRY_ATTEMPTS (10) and _LOCK_RETRY_DELAY_S (0.8s) constants.
Retry loop in _get_memory() retries on Qdrant portalocker errors with
jitter, non-lock errors still fail fast. Add explicit del mem after each
operation to release the lock ASAP. Fix record_failure ordering in search
and add so lock errors are tracked correctly.