Import contacts into a list

Bulk-adds up to 1000 contacts to a list in one call, like the app's CSV import. Double opt-in is skipped: contacts not yet on the list are added as "active" and no verification email is sent, so only import contacts who already agreed to receive your emails. Contacts already on the list (with any status, including "unsubscribed") are left untouched and counted in alreadyOnList. Each row creates the contact if it doesn't exist yet, or updates its fields (name, tags, custom fields) if it does. A row that fails (invalid, suppressed, or disposable email) is reported in errors and does not stop the rest. API key only - a SubscriptionToken is not accepted.