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

FEDERATION.md

@@ -167,7 +167,12 @@ A group is either a **community** or a **circle**.
   and from a community elsewhere once the post's origin answers it gone. A lock sent as it is (`Lock`, Mbin) is taken
   from the post's own server only. A lock (`Announce{Lock}`, or `commentsEnabled: false` on the post) refuses replies,
   ours included, until `Undo{Lock}`. A ban of a persona (`Announce{Block}` with the community as `target`) shows as
-  `blocked_by` on the community and refuses the persona's posts and replies there until the `Undo`.
+  `blocked_by` on the community and refuses the persona's posts and replies there until the `Undo`. A post moved to
+  another community (PieFed's `Announce{Move{object: post, origin, target}}`, relayed by the community it leaves) goes
+  there with its thread; a move into a community this server hosts is not taken.
+- **Feeds** (Lemmy 1.0's multi-communities and PieFed's feeds, `type: Feed`) are read as accounts whose following is
+  their communities: the feed's `following` collection, read once a day with its counts, and each community in it read in
+  turn. A feed cannot be followed from here yet; its communities can.
 
 ## Activities