fix: make notification initialization idempotent

[jjoonleo]

Summary

• Make NotificationService.initialize() idempotent across sequential and concurrent calls.
• Store FCM foreground/opened-app and token-refresh subscriptions so repeated setup does not duplicate notification handling or token refresh registration.
• Document the service-level initialization and listener guarantees in Notification-Behavior.

Tests

• flutter test test/core/services/notification_service_test.dart
• flutter analyze

Closes #532