Submit Dispute Counter

Submit a draft counter's evidence to the payment processor. This is **one-shot and terminal for this round**: the evidence is sent to the processor as your final response, the counter moves to `submitted`, and it can no longer be edited. Only a `draft` counter on a dispute that is currently awaiting a response (e.g. Stripe `needs_response`) can be submitted — otherwise the call returns `400`. Every attached file must both belong to you and have passed a malware scan (`clean`); otherwise the submission is rejected with `422` and nothing is sent. The dispute itself transitions (e.g. to `under_review`) when the processor's webhook confirms receipt, not synchronously here.