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/Support/Harness.cs

@@ -45,6 +45,7 @@ namespace PrivaPub.Tests.Support
 			Groups = new GroupDistributor(Delivery);
 			Records = new ObjectRecords(Queue);
 			Polls = new PollService(Db, Local, Delivery, Queue);
+			Reactions = new Reactions(Db, Delivery);
 			RemotePosts = new RemotePosts(Db, Local, Remote, new NoBlocks(), Queue, Records);
 			Receiver = new InboxReceiver(Local, Remote, Queue, new NoBlocks(), NullLogger<InboxReceiver>.Instance);
 			Processor = new InboxProcessor(Remote, new IActivityHandler[]
@@ -52,8 +53,9 @@ namespace PrivaPub.Tests.Support
 				new FollowHandler(Db, Local, Remote, Delivery),
 				new AcceptHandler(Db, Local),
 				new RejectHandler(Db, Local),
-				new UndoHandler(Db, Local),
-				new LikeHandler(Db),
+				new UndoHandler(Db, Local, Reactions),
+				new LikeHandler(Db, Reactions),
+				new EmojiReactHandler(Db, Reactions),
 				new DislikeHandler(Db),
 				new JoinHandler(Db, Local, Delivery),
 				new AnnounceHandler(Db, Local, RemotePosts, Fanout, Remote),
@@ -93,6 +95,7 @@ namespace PrivaPub.Tests.Support
 		public RemotePosts RemotePosts { get; }
 		public ObjectRecords Records { get; }
 		public PollService Polls { get; }
+		public Reactions Reactions { get; }
 		public TimelineService Timelines { get; }
 		public RelationshipService Relationships { get; }
 		public ReportService Reports { get; }