Likes and boosts arrive, count, notify, and can be taken back

- Like: a Favourite per account and post (unique), the post's counter, a
  Favourite notification for a local author; only on posts the liker
  could see (public and unlisted, or followers-only and direct when they
  were addressed).
- Announce: the original is always refetched from its own origin (or is
  ours), and must be public or unlisted. A boost of a local post counts
  and notifies; a boost by an account a persona follows is stored as a
  reblog row and fanned out to its followers' home timelines (as a reblog,
  where reblogs are wanted). A boost by a stranger of a stranger's post is
  ignored. FEP-1b12 group announces of activities wait for P4.
- Undo of a Like or an Announce reverses each of them; an Undo naming only
  an id is matched against follows, likes and boosts in turn.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

PrivaPub/Federation/Inbox/Handlers/AnnounceHandler.cs

@@ -0,0 +1,106 @@
+using MongoDB.Driver;
+using MongoDB.Entities;
+
+using PrivaPub.Domain.Social;
+using PrivaPub.Domain.Timelines;
+using PrivaPub.Federation.Actors;
+using PrivaPub.Federation.Objects;
+using PrivaPub.Infrastructure.Ids;
+using PrivaPub.Models.Post;
+using PrivaPub.Models.Social;
+using PrivaPub.Models.User;
+using PrivaPub.StaticServices;
+
+using System.Globalization;
+using System.Text.Json.Nodes;
+
+using static PrivaPub.Federation.Objects.ActivityJson;
+
+using PostEntity = PrivaPub.Models.Post.Post;
+
+namespace PrivaPub.Federation.Inbox.Handlers
+{
+	public class AnnounceHandler : IActivityHandler
+	{
+		readonly DbEntities _dbEntities;
+		readonly ILocalActorService _localActors;
+		readonly IRemotePosts _remotePosts;
+		readonly IFanout _fanout;
+
+		public AnnounceHandler(DbEntities dbEntities, ILocalActorService localActors, IRemotePosts remotePosts, IFanout fanout)
+		{
+			_dbEntities = dbEntities;
+			_localActors = localActors;
+			_remotePosts = remotePosts;
+			_fanout = fanout;
+		}
+
+		public string Type => "Announce";
+
+		public async Task Handle(JsonNode activity, ForeignAvatar actor, CancellationToken token)
+		{
+			var inner = activity["object"];
+			if (inner is JsonObject && Value(inner, "type") is "Create" or "Update" or "Delete" or "Like" or "Dislike" or "Undo" or "Add" or "Remove" or "Block")
+				return;
+			var objectUri = Id(inner);
+			var announceId = Id(activity);
+			if (objectUri == default || announceId == default)
+				return;
+			if (await _dbEntities.Posts.Match(p => p.ObjectURI == announceId).ExecuteAnyAsync(token))
+				return;
+
+			var isLocal = objectUri.StartsWith(_localActors.BaseAddress + "/", StringComparison.OrdinalIgnoreCase);
+			var original = await _dbEntities.Posts.Match(p => p.ObjectURI == objectUri && !p.DeletedAt.HasValue && p.ReblogOfPostId == null).ExecuteFirstAsync(token);
+			var followed = await _dbEntities.Followings.Match(f => f.TargetActorURI == actor.ActorURI && f.State == FollowState.Accepted).ExecuteAnyAsync(token);
+			if (original is not { IsFederatedCopy: false } && !followed)
+				return;
+			if (original == default && !isLocal)
+				original = await _remotePosts.StoreContext(objectUri, 0, token);
+			if (original == default || original.Visibility is not (PostVisibility.Public or PostVisibility.Unlisted))
+				return;
+
+			var to = Strings(activity["to"]);
+			var cc = Strings(activity["cc"]);
+			var published = DateTimeOffset.TryParse(Value(activity, "published"), CultureInfo.InvariantCulture, DateTimeStyles.AssumeUniversal, out var at)
+				? at.UtcDateTime
+				: DateTime.UtcNow;
+			var reblog = new PostEntity
+			{
+				ID = PrivacyIds.At(published),
+				ObjectURI = announceId,
+				ActivityURI = announceId,
+				ActorURI = actor.ActorURI,
+				AuthorAccountId = actor.ID,
+				ReblogOfPostId = original.ID,
+				Visibility = Addressing.Classify(to, cc, actor.FollowersURL),
+				To = to,
+				Cc = cc,
+				IsFederatedCopy = true,
+				CreationDate = published,
+				UpdateDate = published
+			};
+			if (reblog.Visibility == PostVisibility.Direct)
+				return;
+			try
+			{
+				await DB.Default.SaveAsync(reblog, token);
+			}
+			catch (MongoWriteException ex) when (ex.WriteError?.Category == ServerErrorCategory.DuplicateKey)
+			{
+				return;
+			}
+
+			await DB.Default.Update<PostEntity>().MatchID(original.ID).Modify(b => b.Inc(p => p.ReblogsCount, 1)).ExecuteAsync(token);
+			if (!original.IsFederatedCopy)
+				await Notifications.Add(original.GroupUserId, NotificationType.Reblog, actor.ID, actor.ActorURI, original.ID, token);
+			await _fanout.Distribute(reblog, token);
+		}
+
+		static List<string> Strings(JsonNode node) => node switch
+		{
+			JsonArray array => array.Select(Id).Where(id => id != default).ToList(),
+			JsonNode single when Id(single) is { } id => new List<string> { id },
+			_ => new List<string>()
+		};
+	}
+}