feat: add cron-expression deferred distribution windows

[Calebstack]

Overview

This PR adds deferred distribution scheduling with cron-expression window definitions, a CronWindowValidator that rejects overlaps with known Stellar maintenance, and persistence so treasury operators can tune settlement cadence without redeploys.

Related Issue

Closes #661

Changes

🗓️ Cron window scheduling

• [ADD] src/db/migrations/025_create_distribution_schedules.sql
  • Per-offering cron + timezone persistence table.
• [ADD] src/services/cronScheduleService.ts (+ tests)
  • Validate-before-persist path; rejects Stellar conflicts and offering overlaps; logs expression diffs.
• [MODIFY] src/services/distributionScheduler.ts (+ tests)
  • Cron gate for deferred runs, 1st-Monday maintenance constraint, year-skip lookahead, offering-scoped window de-dupe, scheduler.window.rejected metrics/logs.
• [MODIFY] Offering / revenue report repositories to read & write cron schedules.

Verification Results

npx jest --runInBand --forceExit \
  src/services/distributionScheduler.test.ts \
  src/services/__tests__/distributionScheduler.test.ts \
  src/services/cronScheduleService.test.ts \
  --coverage=false
✅ passed

Acceptance Criteria	Status
Cron expressions persisted per offering	✅ distribution_schedules + offerings mirror
Reject windows overlapping Stellar maintenance	✅ CronWindowValidator + 1st-Monday constraint
Overlap rejection logs expression diff	✅ scheduler.window.rejected + diff payload
DST / leap-day / year-skip coverage	✅ Tests included

Timeline

• Calebstack committed