Download task artifact

Redirects to a short-lived signed GCS URL for the completed task artifact.

Only for task types that produce a file. transcribe — the only task type today — returns its transcript inline as the result object on GET /v1/tasks/{id}, so this endpoint answers 404 for it: there is no file to download, and no amount of waiting will produce one. A 422 here means the task simply has not finished yet.