Bulk upsert items masterdata. Accepts a JSON array of items. Items are matched by `code` — existing items are updated, new ones are created. Items missing `code` or `name` are silently skipped; validate your payload before sending. Unit identifiers (`inventUnitId`, barcode `unit`) are normalised to lowercase and auto-created if they do not exist. The server processes large arrays in internal batches of 5 000 items. Endpoint timeout is 5 minutes.

NEVER call this endpoint concurrently for the same tenant. Concurrent calls will interfere with each other and produce incorrect results. All requests must be strictly sequential — wait for a 200 OK before sending the next batch.