Billing summary for the caller's billing account

The caller IS the billing account, so this is exact: plan and subscription status both come from their profiles row and the invoices from Stripe. Quota pools at the account, so the plan that sizes the pool lives on the account.

Still prefer GET /v2/workspaces/{id}/billing when you know which workspace you are rendering — it answers the same question about a named workspace's owner.

self_serve_checkout reports whether this deployment can take a payment RIGHT NOW, at the price the pricing page shows. It is true only when a payment processor is wired AND its configured prices reconcile with the published catalog — amount, currency, and monthly interval. When it is false the checkout and portal routes answer 503 not_configured, and a client should hide Upgrade / Manage billing and offer a contact route instead.

It is deliberately NOT just "is Stripe configured": a price edited in the Stripe dashboard takes the button away rather than breaking it, which is the difference between a contact route and a failed checkout.