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

PrivaPub/Models/User/Avatar.cs

@@ -78,6 +78,7 @@ namespace PrivaPub.Models.User
 		public string OutboxURL { get; set; }
 		public string FeaturedURL { get; set; }//featured: the posts it pins
 		public List<PrivaPub.Models.Post.PostFlair> Flairs { get; set; } = new();//a community's labels for its posts
+		public List<string> FeedCommunities { get; set; } = new();//a feed's communities, its `following` (Lemmy's multi-communities, PieFed's feeds)
 		public List<string> SearchableBy { get; set; }//FEP-268d, when the account states it: outranks IsIndexable
 		public string ModeratorsURL { get; set; }//a community's attributedTo (Lemmy, PieFed, Mbin): the collection of its moderators
 		public List<AssertionKey> AssertionKeys { get; set; } = new();//its Ed25519 keys (FEP-521a), which prove what it sends (FEP-8b32)
@@ -152,7 +153,8 @@ namespace PrivaPub.Models.User
 		Group,
 		Organization,
 		Person,
-		Service
+		Service,
+		Feed
 	}
 
 	public enum AvatarAccountState