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/Infrastructure/Http/FederationHttp.cs

@@ -22,6 +22,7 @@ namespace PrivaPub.Infrastructure.Http
 		bool IsAllowed(Uri target);
 		Task<FetchedJson> GetJson(string url, string accept, Action<HttpRequestMessage> sign, CancellationToken token);
 		bool FailedTemporarily(string url);
+		Task<(Uri FinalUri, string Html)> GetPage(string url, CancellationToken token);
 		Task<HttpResponseMessage> Send(HttpRequestMessage request, CancellationToken token);
 		Task<(byte[] Bytes, string ContentType)> GetMedia(string url, long maxBytes, CancellationToken token);
 	}
@@ -181,6 +182,66 @@ namespace PrivaPub.Infrastructure.Http
 			}
 		}
 
+		public async Task<(Uri FinalUri, string Html)> GetPage(string url, CancellationToken token)
+		{
+			if (!Uri.TryCreate(url, UriKind.Absolute, out var target) || !IsAllowed(target))
+				return default;
+			using var timeout = CancellationTokenSource.CreateLinkedTokenSource(token);
+			timeout.CancelAfter(RequestTimeout);
+			try
+			{
+				for (var hop = 0; hop <= MaxRedirects; hop++)
+				{
+					using var request = new HttpRequestMessage(HttpMethod.Get, target);
+					request.Headers.Accept.ParseAdd("text/html, application/xhtml+xml");
+					using var response = await _httpClientFactory.CreateClient(ClientName).SendAsync(request, HttpCompletionOption.ResponseHeadersRead, timeout.Token);
+					if (IsRedirect(response.StatusCode))
+					{
+						var location = response.Headers.Location;
+						var next = location == default ? default : location.IsAbsoluteUri ? location : new Uri(target, location);
+						if (!IsAllowed(next))
+							return default;
+						target = next;
+						continue;
+					}
+					var mediaType = response.Content.Headers.ContentType?.MediaType?.ToLowerInvariant();
+					if (!response.IsSuccessStatusCode || mediaType is not ("text/html" or "application/xhtml+xml"))
+						return default;
+					var bytes = await ReadPrefix(response.Content, MaxPageBytes, timeout.Token);
+					var charset = response.Content.Headers.ContentType?.CharSet?.Trim('"');
+					var encoding = System.Text.Encoding.UTF8;
+					try
+					{
+						if (!string.IsNullOrEmpty(charset))
+							encoding = System.Text.Encoding.GetEncoding(charset);
+					}
+					catch (ArgumentException)
+					{
+					}
+					return (target, encoding.GetString(bytes));
+				}
+				return default;
+			}
+			catch (Exception ex) when (ex is HttpRequestException or BlockedDestinationException or OperationCanceledException && !token.IsCancellationRequested)
+			{
+				_logger.LogInformation("Page {Url} refused: {Reason}", url, ex.Message);
+				return default;
+			}
+		}
+
+		const int MaxPageBytes = 512 * 1024;
+
+		static async Task<byte[]> ReadPrefix(HttpContent content, int limit, CancellationToken token)
+		{
+			await using var stream = await content.ReadAsStreamAsync(token);
+			var buffer = new byte[limit];
+			var total = 0;
+			int read;
+			while (total < limit && (read = await stream.ReadAsync(buffer.AsMemory(total, limit - total), token)) > 0)
+				total += read;
+			return buffer[..total];
+		}
+
 		public async Task<HttpResponseMessage> Send(HttpRequestMessage request, CancellationToken token)
 		{
 			if (!IsAllowed(request.RequestUri))

PrivaPub/Infrastructure/Http/FederationOptions.cs

@@ -6,5 +6,6 @@ namespace PrivaPub.Infrastructure.Http
 		public bool AllowPlainHttp { get; set; }
 		public bool SecureMode { get; set; }
 		public bool AcceptAnyCertificate { get; set; }
+		public bool FetchLinkPreviews { get; set; } = true;//owner decision 1: the server reads linked pages of public posts
 	}
 }