P7: feeds read as accounts, and PieFed's post Move

Lemmy 1.0's multi-communities and PieFed's feeds (`type: Feed`) resolve as accounts: the feed's `following` is kept
once a day with its counts, each community in it is read in turn, and `/api/v1/accounts/:id/following` lists them.
Lemmy's feed carries the instance's key and its text in `description`; both are taken as they are. Following a feed
answers 422 until the owner decides how a feed is followed.

A post moved to another community (PieFed's Move{object: post, origin, target}), relayed by the community it leaves,
goes there with its thread; a move into a community we host is not taken.

Checked in the pasture: a Lemmy 1.0 feed and a PieFed feed list their communities (ours among them); a PieFed post moved
from its web code arrives moved, with its comment.

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

docs/ROADMAP.md

@@ -696,7 +696,10 @@ it, raw where it doesn't.
     server or its moderators collection, kept as `ForeignAvatar.ModeratorsURL`; a resolution is kept on our report);
   - votes in and out; link posts; flairs (**done 2026-10-07**: Lemmy 1.0's `CommunityPostTag` and PieFed's
     `lemmy:CommunityTag`, on posts and communities, described from the community's list, as `privapub.flairs`); `Feed`
-    actors; community polls; post `Move`;
+    actors (**read 2026-10-07**: Lemmy 1.0's and PieFed's feeds resolve as accounts whose `/following` is their
+    communities, kept from the feed's `following` once a day; following one is refused for now, **owner decision
+    pending** on how); community polls; post `Move` (**done 2026-10-07**: PieFed's, relayed by the community the post
+    leaves, moves the post and its thread; checked live);
   - the outbound shape Lemmy requires;
   - communities we host announce to the author's own instance too: **done 2026-10-06** (its server's shared inbox,
     whether anyone there follows the community or not: Lemmy keeps its user's post pending until it is announced back);