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/Models/Social/Downvote.cs

@@ -0,0 +1,13 @@
+using MongoDB.Entities;
+
+namespace PrivaPub.Models.Social
+{
+	public class Downvote : Entity
+	{
+		public string AccountId { get; set; }//ForeignAvatar.ID
+		public string ActorURI { get; set; }
+		public string PostId { get; set; }
+		public string ActivityURI { get; set; }
+		public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+	}
+}