Create a checklist

Creates a new checklist on an event. When a checklist_category_id is supplied, the new checklist's items, view/edit flags, and name are seeded from the chosen category. When it's omitted, an empty checklist named "Checklist" is created with no items.

Required permission: assigned_event_write (when the authenticated user is assigned to the event) or unassigned_event_write (otherwise). If a checklist_category_id is supplied, the category must also be accessible to the user — admins can use any category; non-admin staff can only use categories with allow_staff_view enabled.