P6: link previews read by the server, as the owner decided

- A public post that links to a page gets a preview card: from the post's own data first (FEP-8967 Link preview, the
  object's image, title and summary); otherwise the server reads the page once, 0-60 s after the post arrives, never
  when someone reads it. OpenGraph and Twitter tags give title, description and image; one LinkPreview per address
  is cached for 7 days and shared by the whole server, so a fetch never points at a persona.
- Lemmy link posts, which carry no title or description, get them filled in.
- The page fetch uses the guarded client (public addresses, three redirects, HTML only, first 512 KB).
- Federation:FetchLinkPreviews switches page fetching off.
- Local public posts get cards too.

Checked live: a link to a GoToSocial profile page becomes a card with its title, description and proxied image.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

PrivaPub/Models/Jobs/Job.cs

@@ -25,7 +25,8 @@ namespace PrivaPub.Models.Jobs
 		FetchAncestors,
 		DescribeInstance,
 		PollRefresh,
-		PollClose
+		PollClose,
+		FetchPreview
 	}
 
 	public enum JobState