upsertSchedules

Registers or reconciles code-defined schedules, once per runtime start. Per spec: an unknown ScheduleSpec.id inserts the row with nextFireAt = fireAt; a known id with an EQUAL fingerprint updates nothing (timing state is preserved across restarts); a known id with a DIFFERENT fingerprint overwrites kind, fingerprint and description, sets nextFireAt = fireAt, clears any lease, AND bumps the fence: a claim taken under the old definition must not be completable, because its runs and next fire time were computed from a definition that no longer exists. Last writer wins; concurrent upserts from nodes running different code versions are expected during rolling deploys. Upsert never deletes: a schedule removed from code keeps its row and simply stops being fired, because no node passes its id to dueSchedules.