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

CLAUDE.md

@@ -258,6 +258,9 @@ cd /var/www/privapub.thepra.dev && sudo -u www-data ASPNETCORE_ENVIRONMENT=Produ
 - **A poll vote is a `Note` with a `name`, an `inReplyTo` that is a poll we hold, and no content.** `CreateHandler`
   hands it to `PollService.Receive` before anything else, so it never becomes a reply. Our votes on other servers' polls
   go only to the poll's author, without `published` (PieFed counts a vote only then).
+- **Link previews follow owner decision 1** (`Domain/Content/LinkPreviews.cs`): only public posts, queued on arrival with
+  0–60 s of jitter, one cached `LinkPreview` per address for the whole server, never fetched when someone reads.
+  `LinkPreviews.Wanted` is called where posts are saved (`CreateHandler`, `StoreContext`, `StatusService.Publish`).
 - **A server is described on arrival, never on read.** The first record from a host enqueues `DescribeInstance` (its
   NodeInfo, at most once a week, into `RemoteInstance`), so opening the details view tells nobody anything.
 - **Post ids are the timeline order, so they follow arrival, not `published`.** `PrivacyIds.Arrived` gives a remote post