Backend:
- /api/admin/users: GET list, POST create, PUT update, DELETE
- Admin users can be added with email, display name, and password
- Self-deletion and last-admin deletion are blocked
- Admin role auto-assigned on creation
Frontend:
- AdminUsersTabComponent: list + add/delete admin users
- AdminUserService: HTTP client for admin user CRUD
- AdminUser interface
- 'Admins' tab added to admin dashboard
Tests:
- 10 new tests for admin user management endpoints
- 140 total tests pass (130 original + 10 new)
