P6: emoji reactions in and out

- Incoming reactions in all three shapes: Misskey and Sharkey Likes carrying an emoji (`content` or
  `_misskey_reaction`), Pleroma, Akkoma and Iceshrimp.NET EmojiReacts, and their Undo. Unicode reactions are one
  grapheme; custom ones need their Emoji tag and keep its image; `:name@host:` is read as `name`. A Like whose
  content is a heart stays a favourite.
- Reactions are kept per account and emoji on our posts and on remote ones we hold, and the author of a local post
  gets a `pleroma:emoji_reaction` notification with the emoji.
- Personas react through Pleroma's API (PUT/DELETE /api/v1/pleroma/statuses/:id/reactions/:emoji, GET for the list),
  which sends an EmojiReact (or its Undo) to the post's author.
- Statuses carry `emoji_reactions` (read by Phanpy) and `pleroma.emoji_reactions`, with counts and whether the viewer
  reacted.

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

PrivaPub.Tests/Federation/InboxScenarioTests.cs

@@ -8,6 +8,7 @@ using PrivaPub.Federation.Actors;
 using PrivaPub.Federation.Inbox.Handlers;
 using PrivaPub.Federation.Inbox;
 using PrivaPub.Federation.Moderation;
+using PrivaPub.Domain.Social;
 using PrivaPub.Domain.Statuses;
 using PrivaPub.Federation.Objects;
 using PrivaPub.Federation.Outbox;
@@ -54,7 +55,7 @@ namespace PrivaPub.Tests.Federation
 			_processor = new InboxProcessor(remote, new IActivityHandler[]
 			{
 				new FollowHandler(db, _local, remote, delivery),
-				new UndoHandler(db, _local),
+				new UndoHandler(db, _local, new Reactions(db, delivery)),
 				new CreateHandler(db, _local, remote, delivery, _blocks, new Fanout(db), new RemotePosts(db, _local, remote, _blocks, queue, new ObjectRecords(queue)), new GroupDistributor(delivery), new ObjectRecords(queue), new PollService(db, _local, delivery, queue)),
 				new DeleteHandler(db, _local, remote, delivery, new GroupDistributor(delivery)),
 				new UpdateHandler(db, _local, remote, new GroupDistributor(delivery), new ObjectRecords(queue))