A vote turned the other way replaces the first

Lemmy turns an upvote into a downvote with a Dislike alone (and back with a
Like), so the like stayed counted next to the downvote. Now an account has
one vote on a post: a Dislike takes its like away, a Like its downvote.

The Lemmy scenario's relayed votes were never failing for that reason only:
Lemmy 1.0 sends what it queued every 30 seconds, and the check gave up after
30. It waits a minute now, and both votes are plain checks: 22 pass, the
moderator's removal stays an expected failure (P7).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

PrivaPub/Federation/Inbox/Handlers/DislikeHandler.cs

@@ -49,6 +49,10 @@ namespace PrivaPub.Federation.Inbox.Handlers
 			}
 			Arrival.Accept("stored");
 			await DB.Default.Update<PostEntity>().MatchID(post.ID).Modify(b => b.Inc(p => p.DownvotesCount, 1)).ExecuteAsync(token);
+			// a vote replaces the other one: Lemmy turns an upvote into a downvote with this Dislike alone
+			if (await DB.Default.Find<Favourite>().Match(f => f.PostId == post.ID && f.ActorURI == actor.ActorURI).ExecuteFirstAsync(token) is { } liked
+				&& (await DB.Default.DeleteAsync<Favourite>(liked.ID)).DeletedCount > 0)
+				await DB.Default.Update<PostEntity>().MatchID(post.ID).Modify(b => b.Inc(p => p.FavouritesCount, -1)).ExecuteAsync(token);
 		}
 	}
 }