P5 done: a key we cannot fetch for now gets 503, and follow/like/block ids stay private on purpose

- When a sender's key cannot be fetched because its server timed out or answered 5xx, the inbox answers 503 with
  Retry-After: 300 instead of 401, so Mastodon 4.7 retries rather than switching to RFC 9421 signatures we do not
  verify yet. The fetcher's failure cache now remembers whether a failure was temporary.
- Follow, Like, Block, Accept, Reject and Undo ids are deliberately not dereferenceable: serving them would publish
  who follows, likes and blocks whom. They are always sent with their object embedded.

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

PrivaPub/Federation/Controllers/PeasantsController.cs

@@ -333,6 +333,8 @@ namespace PrivaPub.Federation.Controllers
 		{
 			if (result.Error != default)
 				_logger.LogInformation("Inbox refused with {Status}: {Error}", result.StatusCode, result.Error);
+			if (result.RetryAfterSeconds is { } seconds)
+				Response.Headers.RetryAfter = seconds.ToString(System.Globalization.CultureInfo.InvariantCulture);
 			return result.Error == default ? StatusCode(result.StatusCode) : StatusCode(result.StatusCode, result.Error);
 		}