Start a plan checkout

Creates a Paddle checkout transaction for the requested paid plan and billing interval, and returns the transaction id and a client token for the dashboard to open with Paddle.js as an overlay. The trial is once per account, ever: the backend checks whether the account has any billing history — including a canceled subscription — and selects a trial or no-trial price variant accordingly, so only a genuinely first-ever checkout gets the 14-day card-required trial. The plan change itself lands asynchronously via Paddle's webhook after payment; the dashboard polls GET /me until it appears. Returns 400 validation_failed when billing is not configured on this deployment, when the plan/interval is unknown, when the account already holds the requested plan, or when it already has a live subscription (a subscriber switches plan through POST /billing/change-plan instead).