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.ClientModels/Group/ViewGroup.cs

@@ -8,6 +8,7 @@ namespace PrivaPub.ClientModels.Group
 		public string Description { get; set; }
 		public string Url { get; set; }
 		public string Handle { get; set; }
+		public bool IsCommunity { get; set; }
 		public bool IsDiscoverable { get; set; }
 		public bool ManuallyApprovesMembers { get; set; }
 		public bool IsOwner { get; set; }