Add Notifications Module for In-App Events

[Mkalbani]

Description

Implement a notifications module that delivers in-app messages to users when significant events occur — puzzle solved, achievement unlocked, reward granted, NFT minted. Notifications keep players engaged and informed without requiring them to poll multiple endpoints. The module should support mark-as-read and bulk-clear functionality.

Acceptance Criteria

• A notifications module is scaffolded at src/notifications/
• Notification entity includes id, userId, type, message, isRead, and createdAt
• Notifications are created by internal events from sessions, achievements, and rewards modules
• GET /notifications returns the authenticated user's notifications (unread first)
• PATCH /notifications/:id/read marks a single notification as read
• PATCH /notifications/read-all marks all user notifications as read
• Notifications older than 30 days are soft-deleted automatically
• Unit tests cover creation, retrieval, read-marking, and cleanup