- Extract NotificationItem/NotificationType to Models/Notification.swift
- Create NotificationsServiceProtocol with testable service layer
- Implement markAsRead(id:) and markAllAsRead() with HTTP calls
- Add NotificationError enum with localized descriptions
- Update NotificationsViewModel to use protocol-based service
- Add 18 unit tests (12 ViewModel + 6 Model) with mock service
- Update README with architecture documentation