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/AnnounceHandler.cs

@@ -228,6 +228,15 @@ namespace PrivaPub.Federation.Inbox.Handlers
 				case "Like" or "Dislike" or "Undo":
 					await Relayed(inner, group, token);
 					break;
+				// a moderator features a post in the community, or no longer: the community's own pins
+				case "Add" or "Remove" when Id(inner["target"]) is { } target && target == group.FeaturedURL:
+					var featured = (Relays ?? _services?.GetService(typeof(IEnumerable<IActivityHandler>)) as IEnumerable<IActivityHandler>)
+						?.FirstOrDefault(h => h.Type == Value(inner, "type"));
+					if (featured == default)
+						Arrival.Drop("unsupported");
+					else
+						await featured.Handle(inner, group, token);
+					break;
 				default:
 					Arrival.Drop("unsupported");
 					break;