Update a checklist item

Updates an existing checklist item. Only fields included in the request body are modified — omitted fields remain unchanged. Use this to change the title, notes, due date, checked state, or assigned staff. Toggling checked is also exposed as dedicated /mark_as_checked and /mark_as_unchecked endpoints.

Required permission: assigned_event_write (when the authenticated user is assigned to the item's 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 edited by their owner.