Follow-up to 012-admin-list-views, discovered while building
supporthub-web's own SLA/calendar admin screen (001-agent-admin-ui User
Story 4): holidays could only be added or removed, never read back -
GET /admin/business-calendars/:id returned the bare calendar with no
way to display what holidays were already on file. The repository
already had findByIdWithHolidays; it just wasn't wired to this route.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>