#7333 fix(cron): derive async job retention cutoffs from one clock read

Summary

• The stale-execution cleanup cron computed the async-job retention cutoff and the irrecoverable schedule-carrier cutoff from two separate Date.now() calls, so a millisecond tick between the statements skewed the gap between the two windows by 1 ms.
• Snapshot the clock once and derive both cutoffs from it, which also un-flakes cleanup-stale-executions/route.test.ts (it asserts the gap equals the constant difference).

Type of Change

• Bug fix

Testing

Tested manually — vitest run app/api/cron/cleanup-stale-executions/route.test.ts (15/15), bun run lint, block-registry check, and bun run check:audits (39 audits) all pass.

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)