Create suppliers and their contacts in one call

Creates suppliers and answers 201 with each one as get_supplier_details serves it. The body is {"suppliers": [...]}; every entry needs a name, and may carry the address, currency, min_order_value, lead_time, payment_terms_days, domains, a contacts list and a primary_contact.

A supplier's id is derived from its name, so this call is idempotent on the name and never overwrites. Posting a name that already exists answers the supplier on file, unchanged, with none of the fields in the body applied, and 201 either way. update_supplier is the door for changing a supplier that exists. Two entries with the same name in one body are collapsed to the first.

contacts are created with the supplier and deduplicated on (email, name). A contact with no preferred_contact_method takes email when an email is given and phone otherwise, and in that same case a contact with an email and no name takes the local part of the address, state preferred_contact_method yourself and the name stays null. primary_contact names an existing contact by id and does not create one.

domains is what routes a supplier's mail: a domain like acme.example or a whole address like orders@acme.example. Anything that is neither is refused 400, because a bad entry matches nothing and nothing reports it.

Scope: suppliers:write, which includes suppliers:read.