Upgrade or downgrade an existing subscription

Swaps the price on the organisation’s existing subscription, so a user can never end up paying for two plans at once. For an organisation with no subscription yet, use POST /api/v1/billing/checkout-session.

When the change takes effect depends on direction and cadence, and the response says which:

An at_period_end change means the caller keeps the current plan’s entitlements until the period ends — re-read GET /api/v1/billing/entitlements rather than assuming the new plan applies now.

Broker admins only, and refuses API-token auth.