[NOTIF-01] Implement In-App Notification System

[kilip]

Description

Implement in-app notifications: create, list, and mark as read.

Subtasks

• NOTIF-01-ST-01 Implement NotificationService.create() for inApp channel
• NOTIF-01-ST-02 Build notification list UI (bell icon in top nav)
• NOTIF-01-ST-03 Implement mark as read (isRead: true)
• NOTIF-01-ST-04 Show unread count badge on bell icon

Acceptance Criteria

• NOTIF-01-AC-01 In-app notifications appear in the notification dropdown
• NOTIF-01-AC-02 Unread notifications show a badge count
• NOTIF-01-AC-03 Marking as read updates isRead: true
• NOTIF-01-AC-04 referenceId and referenceType allow navigation to related entity

Technical Notes

• Docs: ERD#25-notifications