SetNodeIdleExpiry

Per-node override of the daemon-wide empty-node grace. The reaper terminates an elastic node that has held no job for the grace period; this makes one node’s grace differ, which is what lets a node vacated by a handover linger long enough to migrate BACK onto, and lets a pre-warmed spare outlive the default.

ZERO means "no override — use the daemon-wide value", so clearing an override and never having set one are the same state. The value is daemon-owned and persisted: it survives a Manager restart, because a restart is exactly when a node that was deliberately kept alive would otherwise be reaped out from under an operator.