Mint a credit top-up checkout link for this product run. Pass pack_id ('small', 'medium' or 'large'). Returns a checkout_url the founder opens to pay; once payment succeeds the Dodo webhook adds the pack's credits to THIS enrollment automatically (no credits are granted here). Founder plan required (on a trial, create_checkout is the path there); scope 'act'. The top-up is always bound to the enrollment your key owns, never one supplied in the request, so it cannot credit another account. Rate limited per product. Inert until credit metering is enabled.