Upload Campaign Brief

Attach a campaign brief the generation call will make the model read.

The upload to the model provider happens here, while the marketer is still in the builder, so an unreadable file is refused at attach time instead of surfacing minutes later as a failed generation.

What comes back is the attachment record's opaque id. The provider file id and the storage location stay on the server, so nothing the browser holds can be edited into a claim on another workspace's document.