feat(listener): add notification retry queue with exponential backoff

[Mrwicks00]

PR: feat(listener): Add notification retry queue with exponential backoff

Branch: feature/notification-retry-queue → main
Closes #25

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Summary

• New NotificationRetryQueue service (listener/src/services/notification-retry-queue.ts) — manages a queue of failed Discord notifications, processes due items on a configurable interval, retries them with exponential backoff, and permanently marks notifications as failed after the max retry limit is reached.
• Integrated into EventSubscriber — when discordService.sendEventNotification() returns false, the event is enqueued for retry instead of silently dropped.
• Configurable via env vars — RETRY_BASE_DELAY_MS (default 5000) and RETRY_MAX_RETRIES (default 5).
• 11 new tests covering enqueue, retry timing, exponential backoff delays, max-retry cutoff, and start/stop behaviour.
• Fixed pre-existing syntax error in discord-notification.ts — a malformed logger.info call with a missing closing brace.

Retry behaviour

Attempt | Delay from previous failure -- | -- 1 | baseDelayMs × 2⁰ = 5s 2 | baseDelayMs × 2¹ = 10s 3 | baseDelayMs × 2² = 20s … | … max | logged as permanently failed

Test plan

•  npm test — 88/88 tests pass across all suites
•  Exponential backoff verified with fake timers (incremental advance + promise flush)
•  Max-retry cutoff removes item from queue and logs error
•  Start/stop idempotency verified
•  No regressions in event-subscriber, discord-notification, or other existing suites

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