Credit purchases
Stripe Checkout creates a one-off payment. Thecheckout.session.completed webhook inserts a credit grant and a positive ledger row.
Studio subscription
Flat-rate $29/mo. AI calls bypass debits but still fire a Stripe meter event for analytics and abuse prevention.
Refunds
AI failure → automatic refund row in ledger with reasonrefund. Studio users receive an analytics-only entry (delta = 0).