Create or update a contact

Creates the contact with its per-channel identities, or attaches to the one that already has one of those identities (the per-channel address is the natural key). Idempotent: repeating the same request answers 200 with created: false instead of failing. attributes are merged with the existing ones (null on a key deletes it) and consent is touched only if declared. 409 if the request's identities belong to different contacts: merging two contacts is irreversible and is not done automatically. On a NEW email identity, warnings flags a suspicious domain typo or an address already undeliverable on SES (suppressed immediately for this tenant): absent, never an empty array, when there is nothing to say. Required scope: contacts:write.