Read the billing catalogue and current plan
Returns the purchasable one-off top-up packs and monthly plans (each with its credits, email fair-use cap and display price in cents), the currentPlanTier the wallet subscribes to (omitted when on no plan), and enabled — false when the deployment has no Stripe secret, meaning every checkout below will return a dry-run stub instead of a payable session. ALWAYS call this before opening a checkout: it is the only source of the valid packId / tierId values, which are deployment config and not guessable. Read-only.
Authorizations
Authorization: Bearer <token>. Accepts EITHER a Keycloak access token (scopes-in-token) OR an opaque POS session token; both resolve to the same pos.* scope vocabulary the route guards enforce.

