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/RemotePosts.cs

@@ -114,7 +114,7 @@ namespace PrivaPub.Federation.Inbox
 		public async Task<PostEntity> StoreContext(string objectUri, int depth, CancellationToken token)
 		{
 			var existing = await _dbEntities.Posts.Match(p => p.ObjectURI == objectUri).ExecuteFirstAsync(token);
-			if (existing != default || depth > MaxDepth)
+			if (existing != default || depth > MaxDepth || await DB.Default.Find<DeletedObject>().Match(d => d.ObjectURI == objectUri).ExecuteAnyAsync(token))
 				return existing;
 
 			using var fetched = await _remoteActors.FetchObject(objectUri, token);