feed.json supports one line summary and long description

I am developing a AppImageHub backend for Discover, the KDE Plasma app store.

Discover shows descriptions in two format:

1. one line short summary (plain text)
2. long description in multiple paragraphs (html)

AppStream supports summary and description fields. However, feed.json doesn't expose summary field.

It would be great if feed.json can support both summary and description.

Start by locating how feed.json is produced and how the existing description is populated. Compare the available AppStream summary and description data with the format expected by Discover. Done means feed.json exposes both a one-line plain-text summary and the existing multi-paragraph description without breaking current consumers.

索引モデルが issue の本文から書いたものです。