Followers and circle members may like, react, vote and downvote

LikeHandler.MaySee let a remote account interact with a followers-only
post only when the post named it in to, cc or its mentions. Our own
followers-only posts are addressed to the followers collection, never to
each follower, and circle posts were not considered at all, so every
like, reaction, downvote and poll vote from a follower on a
followers-only post, and from a member on a circle post, was dropped as
"not-visible". Likes, dislikes, reactions and poll votes now ask
VisibilityPolicy.RemoteCanSee: anyone for public and unlisted posts, an
addressed account for a DM, an accepted follower or an addressed account
for followers-only, a foreign member for a circle post, and never a
server's instance actor (SignedFetchAuthorizer's alias is for refetches
only).

Found by planning the town's multi-server interaction checks (G-0001).

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

PrivaPub/Domain/Privacy/VisibilityPolicy.cs

@@ -1,8 +1,10 @@
 using MongoDB.Entities;
 
+using PrivaPub.Models.Federation;
 using PrivaPub.Models.Group;
 using PrivaPub.Models.Post;
 using PrivaPub.Models.Social;
+using PrivaPub.Models.User;
 
 using System.Linq.Expressions;
 
@@ -47,6 +49,31 @@ namespace PrivaPub.Domain.Privacy
 			};
 		}
 
+		// A remote account may see a post, and so like, react to, vote on or downvote it, when the post was for it: anyone for
+		// public and unlisted, an addressed account for a DM, a follower or an addressed account for followers-only (our own
+		// followers-only posts are addressed to the followers collection, never to each follower), a member for a circle.
+		// Accounts only: the instance-actor alias of SignedFetchAuthorizer.MayRead is for refetches, never for interactions.
+		public static async Task<bool> RemoteCanSee(Post post, ForeignAvatar actor, CancellationToken token)
+		{
+			if (!Shown(post) || actor == default || string.IsNullOrEmpty(actor.ActorURI))
+				return false;
+			if (IsPublicCompiled(post))
+				return true;
+			var addressed = post.To.Contains(actor.ActorURI) || post.Cc.Contains(actor.ActorURI) || post.Mentions.Any(m => m.ActorURI == actor.ActorURI);
+			return post.Visibility switch
+			{
+				PostVisibility.Direct => addressed,
+				//a remote author's followers live on other servers too; we know only who it addressed
+				PostVisibility.FollowersOnly => addressed || !post.IsFederatedCopy && !string.IsNullOrEmpty(post.GroupUserId) && await DB.Default.Find<Follower>()
+					.Match(f => f.LocalActorId == post.GroupUserId && f.LocalActorKind == LocalActorKind.Person && f.IsAccepted && f.ActorURI == actor.ActorURI)
+					.ExecuteAnyAsync(token),
+				PostVisibility.Circle => !string.IsNullOrEmpty(post.GroupId) && await DB.Default.Find<GroupEntity>()
+					.Match(g => g.ID == post.GroupId && g.Members.Any(m => m.IsForeign && m.AvatarId == actor.ActorURI))
+					.ExecuteAnyAsync(token),
+				_ => false
+			};
+		}
+
 		static async Task<bool> FollowsAuthor(string viewerAvatarId, Post post, CancellationToken token)
 		{
 			if (!string.IsNullOrEmpty(post.ActorURI))