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/LikeHandler.cs

@@ -0,0 +1,56 @@
+using MongoDB.Driver;
+using MongoDB.Entities;
+
+using PrivaPub.Domain.Social;
+using PrivaPub.Models.Post;
+using PrivaPub.Models.Social;
+using PrivaPub.Models.User;
+using PrivaPub.StaticServices;
+
+using System.Text.Json.Nodes;
+
+using static PrivaPub.Federation.Objects.ActivityJson;
+
+using PostEntity = PrivaPub.Models.Post.Post;
+
+namespace PrivaPub.Federation.Inbox.Handlers
+{
+	public class LikeHandler : IActivityHandler
+	{
+		readonly DbEntities _dbEntities;
+
+		public LikeHandler(DbEntities dbEntities)
+		{
+			_dbEntities = dbEntities;
+		}
+
+		public string Type => "Like";
+
+		public async Task Handle(JsonNode activity, ForeignAvatar actor, CancellationToken token)
+		{
+			var objectUri = Id(activity["object"]);
+			var post = objectUri == default
+				? default
+				: await _dbEntities.Posts.Match(p => p.ObjectURI == objectUri && !p.DeletedAt.HasValue && p.ReblogOfPostId == null).ExecuteFirstAsync(token);
+			if (post == default || !MaySee(post, actor))
+				return;
+
+			try
+			{
+				await DB.Default.SaveAsync(new Favourite { AccountId = actor.ID, ActorURI = actor.ActorURI, PostId = post.ID, ActivityURI = Id(activity) }, token);
+			}
+			catch (MongoWriteException ex) when (ex.WriteError?.Category == ServerErrorCategory.DuplicateKey)
+			{
+				return;
+			}
+			await DB.Default.Update<PostEntity>().MatchID(post.ID).Modify(b => b.Inc(p => p.FavouritesCount, 1)).ExecuteAsync(token);
+			if (!post.IsFederatedCopy)
+				await Notifications.Add(post.GroupUserId, NotificationType.Favourite, actor.ID, actor.ActorURI, post.ID, token);
+		}
+
+		public static bool MaySee(PostEntity post, ForeignAvatar actor) =>
+			post.Visibility is PostVisibility.Public or PostVisibility.Unlisted
+			|| post.Visibility is PostVisibility.FollowersOnly or PostVisibility.Direct
+				&& (post.To.Contains(actor.ActorURI) || post.Cc.Contains(actor.ActorURI) || post.Mentions.Any(m => m.ActorURI == actor.ActorURI));
+	}
+}