feat: customer sessions and self-serve checkout

[fuzcap]

The SDK side of kaitencloud/kaiten#33: a vendor's backend mints a customer session, and the customer's browser checks out and reads its invoices with it.

Stacked on #4 (feat/public-catalog).

@kaitencloud/server

CustomerSessions.createCustomerSession and CustomerSessions.revokeCustomerSession, generated from the contract like the other resource classes (billing spec §14.6.2). The vendor backend calls it with its ksh_ token, which must hold write:customer_sessions:

const { data } = await CustomerSessions.createCustomerSession({
  client, body: { customerSlug: "acme", instanceSlug: "acme-prod" },
});
// hand data.token to the page; mint a new one before data.expiresAt

@kaitencloud/client

A session module, for a client created with authScheme: "bearer" and a tokenProvider that returns the session. The provider is called on every request, so the host can swap in a fresh token before expiresAt.

• session.previewCheckout(order) → { status: "preview", dueToday, trialEndsAt }. Totals come from the server.
• session.checkout(order) → one of:
  • requires_payment_method: send the customer to setupUrl, then call again with setupSessionId;
  • subscribed, with payment.status ∈ paid | requires_action | failed | processing | not_required.
• session.invoices({ cursor?, limit? }) → the customer's issued invoices, with hostedInvoiceUrl. Poll it after requires_action or processing.
• Shapes follow the SDK's conventions: currency → currencyCode, and a price reads as a PlanPrice.
• With a publishable key, the session methods throw a TypeError before sending anything.

The checkout flow for the React side is §14.6.4: preview (dryRun) → requires_payment_method → hosted setup page (popup, redirect fallback) → second call with setupSessionId → subscribed / requires_action (link to hostedInvoiceUrl) / failed / processing (poll session.invoices()).

Contract snapshot

contracts/openapi.yaml is kaiten billing-step-11's, so most of this diff is generated: the billing surface of steps 2–11 in both packages. The public catalogue now maps the generated types instead of the hand-written ones from #4; its output is unchanged.

Tests

vp run -r build && vp check && vp test: 226 passing. New tests cover:

• the session as bearer;
• the checkout request body and the mapping;
• preview and setup-page answers;
• the invoice list;
• the refusal under a publishable key, with no request sent;
• the server's createCustomerSession.