fix(schedule): make executor lock release single-owner and transactional

[tokenjunkielabs]

Fixes #494

Keeps each claimed schedule on the same PostgreSQL client from the atomic claim through execution bookkeeping and final unlock.

• removes the duplicate finally unlock;
• lets ScheduleService.updateAfterExecution reuse the executor-owned client instead of opening a nested connection/transaction;
• records execution state and clears the claim in one transaction;
• requires locked_by = $podId on the only unlock and aborts if ownership changed;
• leaves failed/unrecorded claims in place for the existing stale-claim recovery instead of clearing another worker's lock.

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