A post whose author cannot be shown is left out of a list, never a 500

The v1.19.0 deploy's tests failed where build.yml's passed. A test stored a public remote post with no author id, and
when the public timeline test ran after it, MastodonMapper.Statuses looked that null up in its accounts and answered
500. The mapper now skips such a post, and the test stores a real author.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ELjqpznMFMNrJoJUj6K5p2

PrivaPub/Api/Mastodon/Mappers/MastodonMapper.cs

@@ -202,8 +202,8 @@ namespace PrivaPub.Api.Mastodon.Mappers
 
 			Status Map(PostEntity post)
 			{
-				if (!accounts.TryGetValue(AuthorOf(post), out var account))
-					return default;
+				if (AuthorOf(post) is not { } author || !accounts.TryGetValue(author, out var account))
+					return default;//a post whose author we cannot show is left out, never a 500
 				var status = new Status
 				{
 					Id = post.ID,