feat(backend): implement unified BullMQ notification queue worker (#506)

[Yusufolosun]

Closes #506

Summary

Implements a dedicated BullMQ-based notification queue worker unifying delivery across email, webhook, and in-app (WebSocket) channels.

Changes

notificationQueue.worker.ts — new worker with:

• Unified deliverEmail, deliverWebhook, and deliverInApp delivery handling
• Central retry policies via retryPolicyService with exponential backoff and jitter
• Priority mapping (critical / high / medium / low → BullMQ queues 1–4)
• Delivery duration and status metrics via MetricsCollector singleton
• In-app broadcasts over WebSocket "alerts" channel via wsServer.broadcastToChannel

index.ts — wired into application lifecycle:

• initNotificationQueueWorker called on startup after the Webhook worker
• stopNotificationQueueWorker called on shutdown before the Webhook worker

notificationQueue.worker.test.ts — unit tests covering:

• enqueueNotification job priorities
• Metric dispatching and channel logic
• Worker lifecycle methods
• Isolated from Redis, DB, and mailer via vi.hoisted mocks

Testing

• ✅ npm run build — 0 type errors
• ✅ npx eslint — clean, no errors or warnings
• ✅ npx vitest run — 8/8 tests pass

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