A remote handle on its own domain, once that domain's WebFinger agrees (FEP-2c59)

An account on a server whose handles are not its host's (Mastodon's LOCAL_DOMAIN apart from WEB_DOMAIN) showed as
user@host. The actor's `webfinger` names the handle; its domain is kept once WebFinger there points back to the actor,
and asked again when the name changes. A persona's blocked servers match a handle's domain as well as the actor's host,
as the lists Mastodon exports name handles' domains.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

PrivaPub/Federation/Actors/RemoteActorService.cs

@@ -190,8 +190,25 @@ namespace PrivaPub.Federation.Actors
 			return true;
 		}
 
-		static async Task<ForeignAvatar> Upsert(ActorDocument actor, ActorKey key, CancellationToken token)
+		// the domain of its handle: its own host, or the one its `webfinger` names (FEP-2c59; a server whose handles are not
+		// its host's, as Mastodon's LOCAL_DOMAIN) once that domain's WebFinger says the handle is this actor. Asked again
+		// only when the handle changes, a rename among them.
+		async Task<string> HandleDomain(ActorDocument actor, CancellationToken token)
 		{
+			var host = new Uri(actor.Id).Authority;
+			var named = actor.WebFinger?.Split('@');
+			if (named is not { Length: 2 } || named[0].Length == 0 || named[1].Length == 0 || named[1].Equals(host, StringComparison.OrdinalIgnoreCase))
+				return host;
+			var domain = named[1].ToLowerInvariant();
+			var stored = await _dbEntities.ForeignAvatars.Match(a => a.ActorURI == actor.Id).ExecuteFirstAsync(token);
+			if (stored?.Domain == domain && stored.UserName == actor.PreferredUsername)
+				return domain;
+			return await ResolveHandle($"{named[0]}@{domain}", token) == actor.Id ? domain : host;
+		}
+
+		async Task<ForeignAvatar> Upsert(ActorDocument actor, ActorKey key, CancellationToken token)
+		{
+			var domain = await HandleDomain(actor, token);
 			var now = DateTime.UtcNow;
 			return await DB.Default.UpdateAndGet<ForeignAvatar>()
 				.Match(a => a.ActorURI == actor.Id)
@@ -199,7 +216,7 @@ namespace PrivaPub.Federation.Actors
 				.Modify(a => a.Name, actor.Name)
 				.Modify(a => a.Biography, ContentSanitizer.Html(actor.Summary))
 				.Modify(a => a.Url, actor.Url)
-				.Modify(a => a.Domain, new Uri(actor.Id).Authority)
+				.Modify(a => a.Domain, domain)
 				.Modify(a => a.InboxURL, actor.Inbox)
 				.Modify(a => a.OutboxURL, actor.Outbox)
 				.Modify(a => a.FollowersURL, actor.Followers)