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/Infrastructure/Http/FederationHttp.cs

@@ -21,6 +21,7 @@ namespace PrivaPub.Infrastructure.Http
 	{
 		bool IsAllowed(Uri target);
 		Task<FetchedJson> GetJson(string url, string accept, Action<HttpRequestMessage> sign, CancellationToken token);
+		bool FailedTemporarily(string url);
 		Task<HttpResponseMessage> Send(HttpRequestMessage request, CancellationToken token);
 		Task<(byte[] Bytes, string ContentType)> GetMedia(string url, long maxBytes, CancellationToken token);
 	}
@@ -107,7 +108,8 @@ namespace PrivaPub.Infrastructure.Http
 					}
 
 					if (!response.IsSuccessStatusCode)
-						return Refuse(negativeKey, url, $"status {(int)response.StatusCode}");
+						return Refuse(negativeKey, url, $"status {(int)response.StatusCode}",
+							transient: (int)response.StatusCode is >= 500 or 429 or 408);
 
 					var mediaType = response.Content.Headers.ContentType?.MediaType;
 					if (mediaType == default || !JsonMediaTypes.Contains(mediaType, StringComparer.OrdinalIgnoreCase))
@@ -125,14 +127,21 @@ namespace PrivaPub.Infrastructure.Http
 			}
 			catch (OperationCanceledException) when (!token.IsCancellationRequested)
 			{
-				return Refuse(negativeKey, url, "a timeout");
+				return Refuse(negativeKey, url, "a timeout", transient: true);
 			}
-			catch (Exception ex) when (ex is HttpRequestException or JsonException or BlockedDestinationException)
+			catch (HttpRequestException ex)
+			{
+				return Refuse(negativeKey, url, ex.Message, transient: true);
+			}
+			catch (Exception ex) when (ex is JsonException or BlockedDestinationException)
 			{
 				return Refuse(negativeKey, url, ex.Message);
 			}
 		}
 
+		public bool FailedTemporarily(string url) =>
+			Uri.TryCreate(url, UriKind.Absolute, out var target) && _cache.TryGetValue(NegativeKey(target), out bool transient) && transient;
+
 		public async Task<(byte[] Bytes, string ContentType)> GetMedia(string url, long maxBytes, CancellationToken token)
 		{
 			if (!Uri.TryCreate(url, UriKind.Absolute, out var target) || !IsAllowed(target))
@@ -200,9 +209,9 @@ namespace PrivaPub.Infrastructure.Http
 
 		static string NegativeKey(Uri target) => "federation-http:refused:" + target.AbsoluteUri;
 
-		FetchedJson Refuse(string negativeKey, string url, string reason)
+		FetchedJson Refuse(string negativeKey, string url, string reason, bool transient = false)
 		{
-			_cache.Set(negativeKey, true, NegativeCacheLifetime);
+			_cache.Set(negativeKey, transient, NegativeCacheLifetime);
 			_logger.LogInformation("GET {Url} refused: {Reason}", url, reason);
 			return default;
 		}