Pins across servers, both ways

An account elsewhere that pins or unpins one of its posts (Add or Remove on its `featured`) now shows those pins on its
profile here (`pinned=true`), in its order and its public posts only; a community's announce of a moderator's Add does
the same for the community. The `featured` collection itself is read with the account's counts, at most once a day, so
pins made before PrivaPub ever saw an account show too. Any other target (Smithereen's wall, a community's moderators)
is dropped. A persona's pin and unpin go to the post's audience as Add and Remove on /trophies, as Mastodon sends them.

Checked live against Mastodon (scenarios/pins.sh, 8 checks). The town's checker learnt three peer rules from the
village: Misskey and Sharkey keep a forwarded reply only with its author's LD signature (only Mastodon signs), they
count no renote by a bot, and Mastodon never sees a Lemmy vote on a post in a community. The village of 2026-10-05
checks clean, 2454 of 2454.

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

PrivaPub/Federation/Inbox/Handlers/FeaturedHandler.cs

@@ -0,0 +1,26 @@
+using PrivaPub.Federation.Actors;
+using PrivaPub.Models.User;
+
+using System.Text.Json.Nodes;
+
+namespace PrivaPub.Federation.Inbox.Handlers
+{
+	// An account pins one of its posts (Add to its `featured` collection) or unpins it (Remove), as Mastodon and Akkoma
+	// send it; inside a community's announce, the community features a post made in it. Any other target (Smithereen's
+	// wall, a community's moderators) is not ours to keep.
+	public class FeaturedHandler : IActivityHandler
+	{
+		readonly IFeaturedPosts _featured;
+		readonly bool _add;
+
+		public FeaturedHandler(IFeaturedPosts featured, bool add)
+		{
+			_featured = featured;
+			_add = add;
+		}
+
+		public string Type => _add ? "Add" : "Remove";
+
+		public Task Handle(JsonNode activity, ForeignAvatar actor, CancellationToken token) => _featured.Receive(activity, actor, _add, token);
+	}
+}