An account that moves shows where it went

PrivaPub dropped Move as an unknown type and showed no `moved` on accounts.
Now a Move is believed as Mastodon believes it: the moving account sends it
about itself, and the new account, read again from its own server, names it
in alsoKnownAs (now kept on remote accounts). The old account then shows the
new one as `moved` in the Mastodon API. The personas following it keep
following it: following the new account on their behalf would tell another
server about them, so that waits for the owner.

Checked live against GoToSocial (scenarios/moves.sh: an alias, a move, 6
checks).

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

PrivaPub/Api/Mastodon/Mappers/MastodonMapper.cs

@@ -166,8 +166,19 @@ namespace PrivaPub.Api.Mastodon.Mappers
 				accounts[avatar.ID] = await Local(_localActors.FromAvatar(avatar), false, token);
 			foreach (var group in await _dbEntities.Groups.Match(g => wanted.Contains(g.ID) && !g.DeletionAt.HasValue && g.Kind == GroupKind.Community).ExecuteAsync(token))
 				accounts[group.ID] = await Local(_localActors.FromGroup(group), false, token);
-			foreach (var foreign in await _dbEntities.ForeignAvatars.Match(f => wanted.Contains(f.ID) && !f.DeletionAt.HasValue).ExecuteAsync(token))
+			var foreigns = await _dbEntities.ForeignAvatars.Match(f => wanted.Contains(f.ID) && !f.DeletionAt.HasValue).ExecuteAsync(token);
+			foreach (var foreign in foreigns)
 				accounts[foreign.ID] = Foreign(foreign);
+			// an account that moved shows where to, once the new account names it as one of its own (as a Move is believed)
+			var movedTo = foreigns.Where(f => !string.IsNullOrEmpty(f.MovedToURL)).Select(f => f.MovedToURL).Distinct().ToList();
+			if (movedTo.Count > 0)
+			{
+				var targets = (await _dbEntities.ForeignAvatars.Match(f => movedTo.Contains(f.ActorURI) && !f.DeletionAt.HasValue).ExecuteAsync(token))
+					.ToDictionary(f => f.ActorURI);
+				foreach (var foreign in foreigns)
+					if (foreign.MovedToURL != default && targets.TryGetValue(foreign.MovedToURL, out var target) && target.AlsoKnownAs.Contains(foreign.ActorURI))
+						accounts[foreign.ID].Moved = Foreign(target);
+			}
 			return accounts;
 		}