Explicitly purchase one face scan using the authenticated customer's wallet, then run it.
Takes the same payload as POST /skin/analyze — calling this endpoint is the user's consent to be charged.

Flow: the configured USD price is resolved, an active subscriber's pay_per_use_discount_percentage
is applied, the amount is converted to the wallet currency, and the wallet is debited. The scan then
runs immediately. If the scan fails the debit is refunded automatically.

Returns 402 with the structured wallet-balance error when the balance is insufficient (no debit, no scan).