A group's owner sees its members and requests, declines and removes

/clientapi/group had no way to see who is in a group or asks to join it,
so a circle's owner could not answer a request from elsewhere. Now:
- GET /clientapi/group/members: the members (local and remote, with their
  role) and the pending requests, for the group's owner and moderators
  only;
- POST /clientapi/group/reject: declines a request, telling the asker's
  server with a Reject of its Follow;
- POST /clientapi/group/remove: takes a member out (never the owner): a
  persona here stops following the group, one elsewhere gets a Reject of
  its Follow, as Mastodon removes a follower.

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

FEDERATION.md

@@ -111,7 +111,8 @@ A group is either a **community** or a **circle**.
   names that member in `cc`, so servers that keep only posts naming one of their accounts (Mastodon, GoToSocial) keep
   it; it names no other member. The posts are served only to a signed request from a member, or from the instance actor
   of a member's server, and that copy names the member (or the members on that server); anyone else gets 404. A
-  Mastodon member's replies reach only the people they mention.
+  Mastodon member's replies reach only the people they mention. Its owner lets a request in with an `Accept` of the
+  Follow, and declines one or takes a member from elsewhere out with a `Reject` of it, as Mastodon removes a follower.
 - Announces from **remote** groups (Lemmy communities) are followed through to the activity: the object is fetched
   from its own origin, never taken from the announce. They are taken from a group someone here follows, and also when
   they relay a vote on one of our posts in a thread of that group (Lemmy sends a vote to the community alone). A post