Set Company Payer

Transfer who pays for this company (PRE-02) — current payer or admin only.

This is how a fiduciary pools their clients onto one account, and the remedy for the payer FK's ON DELETE RESTRICT: a payer who must be deleted transfers their companies first, deliberately, rather than having them silently unmetered.

It moves quota and subscription resolution and nothing else — no membership is created or removed, no principal is widened, and no gate reads the result. Deliberately not in TIER1 and given no UI: it is a rare administrative act.

Runs under the billing elevation (get_billing_scoped), which also refuses a company-scoped API key (ZET-176). The elevation is necessary rather than convenient — a payer is often not a member of the company they pay for — and it grants no authorization: set_payer does the payer-or-admin check itself.

409 if the target user already pays for a company with this one's name. Company names are unique per payer (uq_company_payer_lower_name), so a transfer moves this company into a name-space that may already hold the name; the refusal names the company in the way, because "rename one of the two, then transfer" is the remedy and a bare constraint error would not say which two.