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/Actors/RemoteActorService.cs

@@ -19,6 +19,7 @@ namespace PrivaPub.Federation.Actors
 		Task<FetchedJson> FetchObject(string uri, CancellationToken token);
 		Task<ForeignAvatar> GetActor(string actorUri, bool refresh, CancellationToken token);
 		Task<ForeignAvatar> GetActorByKeyId(string keyId, bool refresh, CancellationToken token);
+		bool KeyTemporarilyUnavailable(string keyId) => false;
 		Task<string> ResolveHandle(string handle, CancellationToken token);
 	}
 
@@ -120,6 +121,8 @@ namespace PrivaPub.Federation.Actors
 			return await Upsert(actor, key, token);
 		}
 
+		public bool KeyTemporarilyUnavailable(string keyId) => Origin.Of(keyId) != default && _http.FailedTemporarily(StripFragment(keyId));
+
 		public async Task<string> ResolveHandle(string handle, CancellationToken token)
 		{
 			var parts = handle?.TrimStart('@').Split('@');