Replace local ownership with durable multi-node ownership when clustered

When session execution may run in a cluster, replace process-local ownership with durable, database-backed ownership. Coordinate work through persisted execution claims containing execution and owner identities, enforce ownership checks before processing or mutation, renew leases while work is active, and reject stale work after ownership changes. Local routing may remain an implementation boundary for single-process execution, but it must not be the source of truth for ownership in multi-node deployments.