CheckoutPreview

FormRequest for POST /v1/billing/preview. The SPA posts this whenever the user toggles the business switch, edits the BTW-ID, or changes the buyer country in the checkout summary, so the reverse-charge legend + tax breakdown stay in lock-step with the server-side computation. The endpoint is read-side and idempotent. `btw_status` is hint-only — the server treats the BTW status as `unverified` whenever the client says `verified` without a cached or fresh VIES match. The wire field exists so the client can preview the rendering on its own state model; the server still owns the decision.