Create and/or update many elements across any of the 22 types in one call.
Each entry in `items` is `{"type": <slug>, "element": <payload>}` with the same
payload shape `create_element` takes: an `element` with an `id` UPDATES that id
(creating it if absent), an `element` without an `id` CREATES a new element.
Items may reference each other by id, including a forward reference to a sibling
later in the list. There is NO delete: bulk_apply never removes an element.
When `atomic` is false (default), items succeed or fail independently and the
response reports each outcome; when `atomic` is true, ANY item failure rolls the
whole batch back and nothing is committed. Up to 1000 items. Requires a WRITE
API-Key. Returns the batch response verbatim: `{errors, items: [{status, id,
created_at, updated_at} | {status, id, error}, ...]}` — `errors` true means at
least one item failed (and, under `atomic`, that nothing committed).