Create or update a contact (upsert by email)

EXPERIMENTAL — enabled account by account; without it every call answers 403 module_disabled. Scopes read:contacts / write:contacts. The email is normalized before matching, so the same person is one contact: only ASCII whitespace (space, TAB, LF, VT, FF, CR) is trimmed from the ends, then it is lowercased — a Unicode space at either end (NBSP, U+3000…) is not trimmed, and the call is 400 invalid_email. attributes is merged shallowly; a new name is registered with the type of its first value. The body is closed: an unknown field is 400 with field (only when the name looks like a field name — no spaces, no @). A contact that was deleted comes back as a NEW contact (201, created: true), from zero: nothing of the deleted one returns, and the unsubscribes the person made still hold.