Conversations are keyed by who is in them, and circles stay home

S8: a direct message joins a conversation only when its participants are
exactly that conversation's members, found through a new
DmGroup.ParticipantsKey (a hash of the sorted members). A remote context
no longer decides anything: it let anyone who knew a conversation's
context post into it, and joining by context while dropping a participant
would have shown a reply to someone it was not addressed to. A context is
kept only when it is on the author's origin. Sending a DM to the same
people again reuses their conversation instead of opening a new one.

S9: Group.Kind is Circle or Community. A circle is not a federated actor:
its actor, collections, WebFinger and inbox answer 404, a remote Follow is
refused, and posts in it are IsLocalOnly - never delivered, never in an
outbox, never served. Communities keep today's behaviour until P4.
Migration _003 makes every existing group a circle, marks their posts
local-only and backfills the conversation keys.

End-to-end inbox tests sign real deliveries from a fake peer: a context
injection, a forged activity id, a note attributed to someone else, a
cross-origin object, a bad signature, junk bodies and a Follow of a circle.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

PrivaPub/Services/PostsService.cs

@@ -65,6 +65,7 @@ namespace PrivaPub.Services
 					return result.Invalidate(_localizer["Avatar not found."], StatusCodes.Status404NotFound);
 
 				LocalActor group = default;
+				var isLocalOnly = false;
 				if (!string.IsNullOrEmpty(form.GroupId))
 				{
 					var groupEntity = await _dbEntities.Groups.MatchID(form.GroupId).ExecuteFirstAsync(token);
@@ -72,6 +73,7 @@ namespace PrivaPub.Services
 						|| !groupEntity.Members.Any(m => !m.IsForeign && m.AvatarId == form.AvatarId))
 						return result.Invalidate(_localizer["Group not found."], StatusCodes.Status404NotFound);
 					group = _localActors.FromGroup(groupEntity);
+					isLocalOnly = !group.IsFederated;
 				}
 
 				var post = new PostEntity
@@ -84,12 +86,19 @@ namespace PrivaPub.Services
 					ContentFormat = ContentFormat.Markdown,
 					HasContentWarning = form.HasContentWarning,
 					AnsweringToPostId = form.AnsweringToPostId,
+					IsLocalOnly = isLocalOnly,
 					ActorURI = author.Uri
 				};
 				post.ID = (string)post.GenerateNewID();
 				post.ObjectURI = author.PostUri(post.ID);
 				await DB.Default.SaveAsync(post, token);
 
+				if (isLocalOnly)
+				{
+					result.Data = ToView(post);
+					return result;
+				}
+
 				var inReplyTo = await ReplyTarget(form.AnsweringToPostId, token);
 				var note = ActivityPubRenderer.Note(post, author, group, inReplyTo);
 				var create = ActivityPubRenderer.Create(author, note, $"create-{post.ID}");
@@ -123,6 +132,8 @@ namespace PrivaPub.Services
 					return result.Invalidate(_localizer["Post not found."], StatusCodes.Status404NotFound);
 
 				await DB.Default.DeleteAsync<PostEntity>(post.ID);
+				if (post.IsLocalOnly)
+					return result;
 
 				var delete = ActivityPubRenderer.Delete(author, post.ObjectURI, $"delete-{post.ID}",
 					new JsonArray(ActivityPubRenderer.Public), new JsonArray(author.Followers));
@@ -205,10 +216,15 @@ namespace PrivaPub.Services
 						if (members.All(m => m.AvatarId != member.AvatarId))
 							members.Add(member);
 					}
-					dmGroup = new DmGroup { Members = members };
-					dmGroup.ID = (string)dmGroup.GenerateNewID();
-					dmGroup.ConversationURI = $"{author.Uri}/conversations/{dmGroup.ID}";
-					await DB.Default.SaveAsync(dmGroup, token);
+					var key = DmGroup.KeyOf(members);
+					dmGroup = await _dbEntities.DmGroups.Match(g => g.ParticipantsKey == key && !g.DeletionAt.HasValue).ExecuteFirstAsync(token);
+					if (dmGroup == default)
+					{
+						dmGroup = new DmGroup { Members = members, ParticipantsKey = key };
+						dmGroup.ID = (string)dmGroup.GenerateNewID();
+						dmGroup.ConversationURI = $"{author.Uri}/conversations/{dmGroup.ID}";
+						await DB.Default.SaveAsync(dmGroup, token);
+					}
 				}
 
 				var dm = new DmPostEntity