FEP-8fcf rests for two weeks after a restore

What a restore lost of either side is not the other server's to mend: for RestoreRecord.FollowersGrace (14 days)
deliveries carry no Collection-Synchronization header, and a follow only the remote remembers is adopted rather than
undone. Without a restore nothing changes; the interop sweep (gts, mastodon, misskey, sharkey, akkoma, pixelfed,
smithereen, followsync, pins, moves) passed, Smithereen's poll check on a second run.

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

PrivaPub/Federation/Actors/FollowingSynchronization.cs

@@ -21,7 +21,8 @@ namespace PrivaPub.Federation.Actors
 	// FEP-8fcf, received: an account elsewhere tells, with its delivery, a digest of its followers here. When the personas
 	// following it here digest otherwise, its partial list (read signed by the instance actor) says who they are: a follow
 	// only PrivaPub remembers ends, a request it answered without our knowing is accepted, and a follow only it remembers
-	// is undone by the persona, as Mastodon does. A follow ends only when the list is the one the digest describes.
+	// is undone by the persona, as Mastodon does, or kept within RestoreRecord.FollowersGrace of a restore, which may have
+	// lost it. A follow ends only when the list is the one the digest describes.
 	public class FollowingSynchronization : IJobHandler
 	{
 		const int MaxPages = 10;
@@ -87,6 +88,19 @@ namespace PrivaPub.Federation.Actors
 				}
 				if (await _localActors.FindByUri(uri, token) is not { Kind: LocalActorKind.Person } stranger)
 					continue;
+				// a follow the restore lost: kept, not undone
+				if (await Infrastructure.Backup.RestoreRecord.InFollowersGrace(token))
+				{
+					await DB.Default.SaveAsync(new Following
+					{
+						AvatarId = stranger.Id,
+						TargetActorURI = actor.ActorURI,
+						TargetAccountId = actor.ID,
+						TargetInboxURL = string.IsNullOrEmpty(actor.InboxURL) ? actor.SharedInboxURL : actor.InboxURL,
+						State = FollowState.Accepted
+					}, token);
+					continue;
+				}
 				// (it has no id for a follow PrivaPub never made; it is undone by actor and object)
 				var undo = new JsonObject
 				{