P5: downvotes, private messages from Lemmy, late Creates of deleted posts, and typed details for clients

- Dislike and its Undo are kept as downvotes (Lemmy, PieFed, Mbin, Friendica) and shown with favourites as votes.
- ChatMessage (Lemmy 0.19, Mbin, PieFed to those two) arrives as a direct message.
- A deleted object's id is remembered for 90 days, so a Create that arrives after its Delete cannot bring the post
  back; the post's ObjectRecord goes with it.
- A Join of one of our objects is answered with Ignore, as FEP-8a8e asks of a server without RSVP.
- A 503 with Retry-After is waited out like a 429 instead of counting as a failure of the host (GoToSocial throttling,
  Mastodon's temporary key failures).
- Status.privapub carries what a Mastodon Status cannot: object type, title, excerpt, cover, the author's source,
  link, video, audio and event details, and up/down votes, with every media URL proxied.

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

PrivaPub/Federation/Inbox/Handlers/JoinHandler.cs

@@ -0,0 +1,48 @@
+using PrivaPub.Federation.Actors;
+using PrivaPub.Federation.Outbox;
+using PrivaPub.Federation.Rendering;
+using PrivaPub.Models.Federation;
+using PrivaPub.Models.User;
+using PrivaPub.StaticServices;
+
+using System.Text.Json.Nodes;
+
+using static PrivaPub.Federation.Objects.ActivityJson;
+
+namespace PrivaPub.Federation.Inbox.Handlers
+{
+	public class JoinHandler : IActivityHandler
+	{
+		readonly DbEntities _dbEntities;
+		readonly ILocalActorService _localActors;
+		readonly IDeliveryService _delivery;
+
+		public JoinHandler(DbEntities dbEntities, ILocalActorService localActors, IDeliveryService delivery)
+		{
+			_dbEntities = dbEntities;
+			_localActors = localActors;
+			_delivery = delivery;
+		}
+
+		public string Type => "Join";
+
+		public async Task Handle(JsonNode activity, ForeignAvatar actor, CancellationToken token)
+		{
+			var objectUri = Id(activity["object"]);
+			if (objectUri == default || !objectUri.StartsWith(_localActors.BaseAddress + "/", StringComparison.OrdinalIgnoreCase))
+				return;
+			var post = await _dbEntities.Posts.Match(p => p.ObjectURI == objectUri && !p.IsFederatedCopy && !p.DeletedAt.HasValue).ExecuteFirstAsync(token);
+			var owner = post == default ? default : await _localActors.FindById(LocalActorKind.Person, post.GroupUserId, token);
+			if (owner == default || string.IsNullOrEmpty(actor.InboxURL))
+				return;
+			await _delivery.Enqueue(owner, new[] { actor.InboxURL }, new JsonObject
+			{
+				["@context"] = ActivityPubRenderer.ActivityStreams,
+				["id"] = owner.ActivityUri($"ignore-{Guid.NewGuid():N}"),
+				["type"] = "Ignore",
+				["actor"] = owner.Uri,
+				["object"] = activity.DeepClone()
+			}, token);
+		}
+	}
+}