P5: downvotes, private messages from Lemmy, late Creates of deleted posts, and typed details for clients

- Dislike and its Undo are kept as downvotes (Lemmy, PieFed, Mbin, Friendica) and shown with favourites as votes.
- ChatMessage (Lemmy 0.19, Mbin, PieFed to those two) arrives as a direct message.
- A deleted object's id is remembered for 90 days, so a Create that arrives after its Delete cannot bring the post
  back; the post's ObjectRecord goes with it.
- A Join of one of our objects is answered with Ignore, as FEP-8a8e asks of a server without RSVP.
- A 503 with Retry-After is waited out like a 429 instead of counting as a failure of the host (GoToSocial throttling,
  Mastodon's temporary key failures).
- Status.privapub carries what a Mastodon Status cannot: object type, title, excerpt, cover, the author's source,
  link, video, audio and event details, and up/down votes, with every media URL proxied.

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

PrivaPub/Federation/Inbox/Handlers/DislikeHandler.cs

@@ -0,0 +1,47 @@
+using MongoDB.Driver;
+using MongoDB.Entities;
+
+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 DislikeHandler : IActivityHandler
+	{
+		readonly DbEntities _dbEntities;
+
+		public DislikeHandler(DbEntities dbEntities)
+		{
+			_dbEntities = dbEntities;
+		}
+
+		public string Type => "Dislike";
+
+		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 || !LikeHandler.MaySee(post, actor))
+				return;
+
+			try
+			{
+				await DB.Default.SaveAsync(new Downvote { 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.DownvotesCount, 1)).ExecuteAsync(token);
+		}
+	}
+}