fix: coordinate transaction reminder cron across multiple instances

[Okorie2000-code]

Summary

Prevents duplicate reminder notifications when multiple application instances run the transaction-reminders cron job simultaneously.

Coordination strategy: Redis distributed lock

sendDeadlineReminders now acquires a Redis lock (lock:transaction-reminders) via atomic SET NX EX before processing begins. Any instance that cannot acquire the lock logs a message and returns early — no reminders are sent twice.

• Lock key: lock:transaction-reminders
• TTL: 300 s (5 min — exceeds expected job runtime; prevents stale locks on crash)
• Release: finally block — released on both success and error
• Infrastructure reused: CacheService.setNx added to the existing CacheService; no new packages

Files changed

File	Change
src/cache/cache.service.ts	Added setNx(key, value, ttlSeconds) — atomic Redis SET NX EX
src/transactions/transaction-reminders.service.ts	Lock acquire/release wrapping processReminders
src/transactions/transaction-reminders.service.spec.ts	New: full test coverage for lock scenarios + regression
src/transactions/transactions.module.ts	CacheModuleConfig import (already present, no change needed)

Tests

• ✅ Lock acquired → reminders processed normally
• ✅ Lock unavailable → processing skipped, no notifications sent
• ✅ Lock released after success
• ✅ Lock released after processing error
• ✅ Regression: buyer/seller notifications, opt-out, dedup, milestone update

Verification

npm test   → 9/9 pass
npm run lint  → 0 errors
npm run build → clean

Closes #696

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