Start a hosted Square checkout for a paid tier

Returns a Square-hosted payment URL for the requested tier. No API key is needed — a prospect has not got one yet. The email Square collects becomes the account, so a key issued later for the same address inherits the paid quota. Enterprise returns a contact route rather than a payment link. THIS IS THE PATH FOR SOMEONE WHO IS NOT YET PAYING. It starts a NEW Square recurring subscription, so it is refused with 409 conflict (details.reason existing_subscription) when the request carries a dashboard session for an account that already has a live plan on this API — a second subscription would be charged alongside the first, every month, and this API cannot cancel either one. Change tier with POST /v1/subscription/plan, or, when Square collects your base fee, cancel it with Square first.