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

docs/ROADMAP.md

@@ -458,9 +458,11 @@ it, raw where it doesn't.
   - then advertise `api_versions.mastodon ≥ 7` (4.5.0).
 - **Emoji reactions** in all three inbound forms, plus outbound `EmojiReact`, exposed as `emoji_reactions` (done in
   v1.11.0; Pleroma's `/api/v1/pleroma/statuses/:id/reactions` endpoints and `pleroma:emoji_reaction` notifications).
-- **Link cards:**
+- **Link cards** (done in v1.12.0):
   - from the object, or from FEP-8967 `preview`, without fetching;
-  - fetching the page itself is the owner's decision (INTEROP §6.1).
+  - otherwise the server reads the page (owner decision 1): public posts only, 0–60 s after arrival, cached per address
+    for 7 days across the whole server, `Federation:FetchLinkPreviews` to switch it off. Checked live against a
+    GoToSocial profile page.
 - **Media:**
   - video playback through the proxy (Range requests, HLS playlist rewriting, the poster), with a `video` card;
   - audio attachments;