SessionExecution fixed-expiry heartbeat

Trap: a repeated heartbeat with Date.now() captured outside its body, or a lease sized only for renewal cadence, looks sufficient because SQL still runs periodically. It can write a fixed expiry or let bounded node clock skew expose healthy work. Fix: recompute time each tick, require the exact claim to remain unexpired, never revive an expired claim, and keep lease duration above maximum clock skew plus one renewal interval.