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/Federation/Inbox/Handlers/CreateHandler.cs

@@ -4,6 +4,7 @@ using MongoDB.Entities;
 using PrivaPub.Domain.Timelines;
 using PrivaPub.Federation.Actors;
 using PrivaPub.Federation.Moderation;
+using PrivaPub.Domain.Content;
 using PrivaPub.Domain.Statuses;
 using PrivaPub.Federation.Objects;
 using PrivaPub.Federation.Outbox;
@@ -37,10 +38,12 @@ namespace PrivaPub.Federation.Inbox.Handlers
 		readonly IGroupDistributor _groups;
 		readonly IObjectRecords _records;
 		readonly IPollService _polls;
+		readonly ILinkPreviews _previews;
 
 		public CreateHandler(DbEntities dbEntities, ILocalActorService localActors, IRemoteActorService remoteActors, IDeliveryService delivery,
-			IDomainBlocks domainBlocks, IFanout fanout, IRemotePosts remotePosts, IGroupDistributor groups, IObjectRecords records, IPollService polls)
+			IDomainBlocks domainBlocks, IFanout fanout, IRemotePosts remotePosts, IGroupDistributor groups, IObjectRecords records, IPollService polls, ILinkPreviews previews)
 		{
+			_previews = previews;
 			_records = records;
 			_polls = polls;
 			_groups = groups;
@@ -148,6 +151,7 @@ namespace PrivaPub.Federation.Inbox.Handlers
 				return;
 			}
 			await _records.Record(note, post, ObjectPath.Delivered, refetched, token);
+			await _previews.Wanted(post, token);
 
 			if (parent != default)
 				await DB.Default.Update<PostEntity>().MatchID(parent.ID).Modify(b => b.Inc(p => p.RepliesCount, 1)).ExecuteAsync(token);

PrivaPub/Federation/Inbox/RemotePosts.cs

@@ -1,6 +1,7 @@
 using MongoDB.Driver;
 using MongoDB.Entities;
 
+using PrivaPub.Domain.Content;
 using PrivaPub.Federation.Actors;
 using PrivaPub.Federation.Moderation;
 using PrivaPub.Federation.Objects;
@@ -39,10 +40,12 @@ namespace PrivaPub.Federation.Inbox
 		readonly IDomainBlocks _domainBlocks;
 		readonly IJobQueue _queue;
 		readonly IObjectRecords _records;
+		readonly ILinkPreviews _previews;
 
 		public RemotePosts(DbEntities dbEntities, ILocalActorService localActors, IRemoteActorService remoteActors, IDomainBlocks domainBlocks,
-			IJobQueue queue, IObjectRecords records)
+			IJobQueue queue, IObjectRecords records, ILinkPreviews previews)
 		{
+			_previews = previews;
 			_records = records;
 			_dbEntities = dbEntities;
 			_localActors = localActors;
@@ -143,6 +146,7 @@ namespace PrivaPub.Federation.Inbox
 				return await _dbEntities.Posts.Match(p => p.ObjectURI == objectUri).ExecuteFirstAsync(token);
 			}
 			await _records.Record(note, post, ObjectPath.Fetched, refetched: false, token);
+			await _previews.Wanted(post, token);
 
 			if (grandparent == default && !string.IsNullOrEmpty(note.InReplyTo) && depth < MaxDepth)
 				await _queue.Enqueue(JobKind.FetchAncestors, JsonSerializer.Serialize(new AncestorsPayload(post.ID, depth + 1)), new Uri(note.InReplyTo).Host,