Bulk create contacts

Import up to 1000 contacts at a time. Skips duplicates, merging any new tags onto the existing contact. accountId is required whenever contacts carry a platformIdentifier (or a row-level accountId); platform is always derived from the resolved account, never used to decide whether channels are created, and a mismatched platform 404s as account not found. When accountId is set, each contact must carry a platformIdentifier; a row missing it is rejected individually (reported in errors[], HTTP 200), not a 400 for the whole import. On phone platforms (whatsapp, sms) the platformIdentifier is normalized to digits and a value that is not phone-shaped is rejected per contact and reported in errors[], not imported.