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/UndoHandler.cs

@@ -40,6 +40,26 @@ namespace PrivaPub.Federation.Inbox.Handlers
 				await UndoLike(inner, innerId, actor, token);
 			if (innerType is null or "Announce")
 				await UndoAnnounce(innerId, actor, token);
+			if (innerType is null or "Dislike")
+				await UndoDislike(inner, innerId, actor, token);
+		}
+
+		async Task UndoDislike(JsonNode inner, string innerId, ForeignAvatar actor, CancellationToken token)
+		{
+			var downvote = innerId == default
+				? default
+				: await DB.Default.Find<Downvote>().Match(d => d.ActivityURI == innerId && d.ActorURI == actor.ActorURI).ExecuteFirstAsync(token);
+			if (downvote == default && inner is JsonObject && Id(inner["object"]) is { } objectUri)
+			{
+				var post = await _dbEntities.Posts.Match(p => p.ObjectURI == objectUri).ExecuteFirstAsync(token);
+				downvote = post == default
+					? default
+					: await DB.Default.Find<Downvote>().Match(d => d.PostId == post.ID && d.ActorURI == actor.ActorURI).ExecuteFirstAsync(token);
+			}
+			if (downvote == default)
+				return;
+			await DB.Default.DeleteAsync<Downvote>(downvote.ID);
+			await DB.Default.Update<PostEntity>().MatchID(downvote.PostId).Modify(b => b.Inc(p => p.DownvotesCount, -1)).ExecuteAsync(token);
 		}
 
 		async Task UndoFollow(JsonNode inner, string innerId, ForeignAvatar actor, CancellationToken token)