Expose guest notifications as MCP tools

[DmyMi]

Description

Expose notification inspection and notification-preference APIs through MCP.

MCP tools

• list_my_notifications
• mark_notification_read
• mark_all_notifications_read
• get_notification_preferences
• update_notification_preferences

MCP resources

• sharebite://guest/notification-types
• sharebite://guest/notification-preferences-schema

Acceptance criteria

• MCP client can list authenticated user notifications.
• Read/unread updates are supported.
• Notification types are documented as MCP resources.
• No cross-user notification access is possible.
• Tests cover pagination, unread filtering, mark-as-read, and unauthorized access.