Label many documents at once

Adds and/or removes labels on up to 200 documents in one call — the selection bar’s «Etichette», for a job like «etichetta tutte le fatture di Bianchi»: find the documents with documents_search, then pass their ids here instead of calling documents_set_labels once per document. Name each label by its id (from documents_list_labels) or by its exact name; a name two labels share is refused with the candidates. Removal runs before addition, so a label in both lists ends up ON the documents. Refused whole — nothing changes — when one of the documents is in a class the caller may only read. An id that is not a document the caller can see is skipped, and the answer says how many were reached. The records themselves never change.