feat(admin-notifications): add in-app notification management

[ibourgeois]

Goal

Enable admins to send in-app notifications to all users, a role, or specific users, and let recipients view them in the app.

Scope

• Database-backed in-app notifications
• Admin UI to compose and target notifications
• End-user notification center or inbox UI

Out of Scope

• Email, SMS, or push delivery channels
• Rich campaign analytics

Acceptance Criteria

• Admins can send notifications to all users, a role, or selected users
• Users can view and manage read state for in-app notifications
• Notification targeting and display are covered by tests

Dependencies

• Depends on feat(admin): add protected admin area shell and dashboard entry #11 and feat(admin): add first-admin bootstrap command #12

Test Plan

• Add notification delivery tests for each targeting mode
• Add UI or feature coverage for notification listing and read state