feat: Push Notification API for Learning Events

[prodbycorne]

This pull request implements the Push Notification API for real-time learner updates as described in issue #53.

Changes

Database Schema (prisma/schema.prisma)

• Added DeviceToken model for per-user, per-platform FCM token storage
• Added NotificationPreference model for controlling which event types a user receives (rewardReceipt, quizPassFail, streakReminders)
• Added NotificationLog model for tracking push delivery attempts with retry and dead-letter handling

Notification Service (src/services/notification.service.ts)

• Device token registration with upsert logic
• Notification preference management (defaults to enabled for all types)
• Queue-based notification dispatch with user preference checks
• Firebase Cloud Messaging (FCM) integration via firebase-admin
• Exponential backoff retry mechanism (1min, 5min, 25min...)
• Dead-letter handling after 5 failed attempts
• Graceful fallback when Firebase is not configured (dev/test environments)

Notification Controller and Routes

• POST /v1/notifications/devices -- Register a device token (authenticated)
• PATCH /v1/notifications/preferences -- Update notification preferences (authenticated)
• GET /v1/notifications/delivery-status -- Query delivery logs for troubleshooting (authenticated)
• Zod validation on all inputs
• OpenAPI documentation annotations on all endpoints

Event Triggers

• Quiz pass/fail notifications fired from module.controller.ts after module completion
• Reward receipt notifications fired from reward.service.ts after successful reward payout
• All triggers are non-blocking (fire-and-forget with error logging)

Tests (18 new tests, all passing)

• tests/notification.controller.test.ts -- 10 tests covering auth guards, validation, and all endpoint paths
• tests/notification.service.test.ts -- 8 tests covering token registration, preference checks, queue logic, retry behavior, and dead-letter handling

Closes #53

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[Kaylahray]

Hi please resolve conflicts @prodbycorne

[Kaylahray]

Hi @prodbycorne

[prodbycorne]

Addressed the remaining CI failure on this PR by fixing the lint error in src/config/database.ts (newline-before-return).\n\nCurrent blocker is that the latest workflow run is in action_required state (no jobs started), so a maintainer needs to approve/re-run the workflow for forked PRs. Once that is done, checks should execute on commit 79e71f6.