Create a checklist item

Adds a new item to a checklist. When checklist_id is supplied, the item is added to that checklist. When it's omitted, the item is added to the franchise-wide "Shared Checklist" (created on first use).

Required permission: assigned_event_write (when the authenticated user is assigned to the target checklist's event) or unassigned_event_write (otherwise). Staff users without admin role need the per-checklist allow_staff_edit flag instead; customers need allow_customer_edit. Items on user-checklists (personal task lists) can only be created by their owner.