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.Tests/Domain/LinkPreviewTests.cs

@@ -0,0 +1,43 @@
+using PrivaPub.Domain.Content;
+using PrivaPub.Models.Post;
+
+namespace PrivaPub.Tests.Domain
+{
+	public class LinkPreviewTests
+	{
+		[Fact]
+		public void Reads_open_graph_then_twitter_then_the_page_title()
+		{
+			var preview = LinkPreviews.Read("https://news.example/a?utm_source=x", new Uri("https://news.example/a"), """
+				<html><head><title>Fallback title</title>
+				<meta property="og:title" content="Rain &amp; sun">
+				<meta name="twitter:description" content="A <b>long</b> story">
+				<meta property="og:image" content="/img/cover.jpg">
+				<meta property="og:site_name" content="News">
+				</head><body></body></html>
+				""");
+
+			Assert.Equal("Rain & sun", preview.Title);
+			Assert.Equal("A long story", preview.Description);
+			Assert.Equal("https://news.example/img/cover.jpg", preview.ImageURL);
+			Assert.Equal("News", preview.SiteName);
+			Assert.False(preview.Failed);
+			Assert.True(LinkPreviews.Read("https://x.example", new Uri("https://x.example"), "<html><body>nothing</body></html>").Failed);
+		}
+
+		[Fact]
+		public void Picks_the_first_real_link_and_skips_mentions_tags_quotes_and_ourselves() =>
+			Assert.Equal("https://blog.example/post", LinkPreviews.FirstLink(new Post
+			{
+				Url = "https://m.example/@a/1",
+				ContentHtml = """
+					<p><span class="h-card"><a href="https://m.example/@bob" class="u-url mention">@bob</a></span>
+					<a href="https://m.example/tags/x" class="mention hashtag" rel="tag">#x</a>
+					<span class="quote-inline">RE: <a href="https://k.example/notes/1">link</a></span>
+					<a href="https://privapub.test/@me">me</a>
+					<a href="https://m.example/@a/1">self</a>
+					<a href="https://blog.example/post">read this</a></p>
+					"""
+			}, "https://privapub.test"));
+	}
+}