Artifacts support uploading multiple files at once

What

• A new asset_uploads result type (alongside the existing singular asset_upload) supports uploading multiple files in one batch.
• Each file in the batch is reported individually as uploaded (with id, url, size, content type, and sha256), failed, or not attempted, with a reason/message and an optional may_be_stored flag.
• The UI now shows a summary line like "uploaded N of M files" followed by a per-file status list showing the URL, "not attempted", or "failed" for each file.
• A new asset_uploads permission-scope entry was added alongside the existing singular asset_upload scope.

Why

This lets Artifacts upload several files in a single operation with clear per-file feedback, instead of requiring one upload call and result per file.