Import contacts from CSV

Import up to 5,000 rows (5 MB) per request. The header row needs an email column; name (or first_name and last_name), status, and tags (separated by ; or |) are recognized and other columns become custom fields. consent_basis is required and is recorded on every new contact. Optional tags are added to every accepted row. Rows whose status says unsubscribed import as unsubscribed. Existing contacts keep their status; set update_existing to merge names and fields. Duplicate rows are skipped and invalid rows are reported by line.