Upload a file as an asset to an existing release.
Completes the publishing workflow for extension .zip packages.
Uses multipart form POST to Gitea's release assets endpoint.
Safety: 100MB cap, requires absolute file_path, supports custom
asset name (defaults to filename basename).
Closes the gap in the release workflow:
1. create_release (existing) - creates the release
2. upload_release_attachment (NEW) - uploads the .zip/binary asset
