Update web packages

Update up to BATCH_MAX_ITEMS web packages in a single request.

title is the only writable attribute. type, state and base_path are server-owned, and display_name is derived from the title, state and archive filename — a client that sends any of them has them silently ignored rather than rejected.

Note display_name only reflects title once the package reaches READY. While a package is PROCESSING or ERROR its display name is the state and filename, so a title change appears to have no effect on it until ingestion finishes. That is WebPackage.build_display_name's behaviour, unchanged by this API.

Returns 200 if all items succeed, 207 if any fail, 400 if the envelope is empty or over the cap.

Required OAuth scope: web-packages:write