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/Middleware/SocialPubConfigurations.cs

@@ -77,6 +77,8 @@ namespace PrivaPub.Middleware
 				.AddSingleton<IActivityHandler, UndoHandler>()
 				.AddSingleton<IActivityHandler, LikeHandler>()
 				.AddSingleton<IActivityHandler, DislikeHandler>()
+				.AddSingleton<IActivityHandler, EmojiReactHandler>()
+				.AddSingleton<IReactions, Reactions>()
 				.AddSingleton<IActivityHandler, JoinHandler>()
 				.AddSingleton<IActivityHandler, AnnounceHandler>()
 				.AddSingleton<IActivityHandler, FlagHandler>()