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

FEDERATION.md

@@ -128,6 +128,18 @@ Received `summary` is a content warning only on a `Note` or `Question`, or when
 Visibility is expressed in `to`/`cc` the way Mastodon does it: public, unlisted, followers-only and direct. Inbound
 followers-only posts are recognised by the author's own `followers` collection.
 
+## Link previews
+
+For a public post that links to a page, PrivaPub builds a preview card. It uses, in order:
+1. a Link attachment's own `preview` (FEP-8967) or the post's own image, title and summary;
+2. otherwise, the linked page, read once by the server between 0 and 60 seconds after the post arrives (never when
+   someone reads it), with an `Accept: text/html` request that reads at most 512 KB, through the same address checks as
+   every other fetch.
+
+The page's OpenGraph and Twitter tags give the title, description and image; the result is cached per address for 7
+days and shared by every account on the server. Images are served to clients only through PrivaPub's media proxy.
+`Federation:FetchLinkPreviews=false` turns page fetching off.
+
 ## Local-only posts
 
 Posts with a location (shown to nearby users of this server) never leave the server, in any form.