Validate external file URL

Pre-flight check of an audio file by link, without creating an order: the file is fetched, its duration measured, and the speech and content checks an order goes through are run on it. A check that cannot run on our side is reported as unavailable (available: false, a null finding) and never makes the file invalid. A link that cannot be fetched, a file over 750 MB or audio that cannot be read answers 400 with valid: false and an error code. Not offered until the account email is confirmed (403 email_verification_required): submit directly, the same checks run and nothing is charged if the audio is refused.