AP: use `preview` field?

[snarfed]

AS2 has a preview field! It didn't really get used in the wild in the fediverse much, so we didn't use it, but we're starting to see conversation (and implementation!) around long form content, eg Ghost emits it. Notably though, it's the same as summary. Here's an example:

{
  "type": "Article",
  "id": "https://www.404media.co/.ghost/activitypub/article/b103df60-dcff-4225-93c9-f8ce191476dc",
  "url": "https://www.404media.co/doj-deletes-study-showing-domestic-terrorists-are-most-often-right-wing/",
  "name": "...",
  "image": "https://www.404media.co/content/images/2025/09/pam-bondi.jpeg",
  "content": "...",
  "attributedTo": "https://www.404media.co/.ghost/activitypub/users/index"
  "preview": {
    "type": "Note",
    "id": "https://www.404media.co/.ghost/activitypub/note/5639516",
    "content": "..."
  },
  "summary": "...",
  "published": "2025-09-16T16:25:27Z",
  "updated": "2025-09-16T16:25:27Z",
  "..."
}

We could consider adding it to Articles and/or Notes with no content of their own.

cc @anujahooja

[snarfed]

snarfed/granary@ab4eacb, snarfed/granary@81c877a