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.Tests/Http/MastodonFeedTests.cs

@@ -0,0 +1,67 @@
+using PrivaPub.Federation.Actors;
+using PrivaPub.Models.Jobs;
+using PrivaPub.Tests.Support;
+using PrivaPub.Tests.Support.Host;
+
+using System.Net;
+using System.Text.Json.Nodes;
+
+namespace PrivaPub.Tests.Http
+{
+	// Lemmy 1.0's multi-communities and PieFed's feeds are `Feed` actors whose `following` is their communities
+	[Trait("Category", "Integration")]
+	public sealed class MastodonFeedTests : IAsyncLifetime
+	{
+		PrivaPubHost _host;
+		Peer _peer;
+
+		public async ValueTask InitializeAsync()
+		{
+			Assert.SkipUnless(MongoFixture.Enabled, MongoFixture.Skip);
+			_host = await PrivaPubHost.Shared();
+			_peer = await Peer.Start();
+		}
+
+		public async ValueTask DisposeAsync()
+		{
+			if (_peer != default)
+				await _peer.DisposeAsync();
+		}
+
+		[Fact]
+		public async Task A_feed_shows_as_an_account_whose_following_is_its_communities()
+		{
+			var token = TestContext.Current.CancellationToken;
+			var reader = await _host.Mastodon("feeder");
+			var cats = new RemoteActor(_peer, "cats", type: "Group");
+			var dogs = new RemoteActor(_peer, "dogs", type: "Group");
+			var name = $"feed{Guid.NewGuid():N}"[..12];
+			var feed = $"{_peer.A}/m/{name}";
+			// as Lemmy serves one: the instance's inbox and key, its text in `description`
+			_peer.Serve($"/m/{name}", new JsonObject
+			{
+				["@context"] = new JsonArray("https://join-lemmy.org/context.json", "https://www.w3.org/ns/activitystreams"),
+				["type"] = "Feed", ["id"] = feed, ["inbox"] = $"{_peer.A}/inbox",
+				["publicKey"] = new JsonObject { ["id"] = $"{_peer.A}/#main-key", ["owner"] = $"{_peer.A}/", ["publicKeyPem"] = "-----BEGIN PUBLIC KEY-----\n-----END PUBLIC KEY-----\n" },
+				["following"] = $"{feed}/following", ["preferredUsername"] = name, ["name"] = "Pets", ["description"] = "every pet",
+				["attributedTo"] = $"{_peer.A}/users/owner"
+			}.ToJsonString());
+			_peer.Serve($"/m/{name}/following", new JsonObject
+			{
+				["type"] = "Collection", ["id"] = $"{feed}/following", ["totalItems"] = 2, ["items"] = new JsonArray(cats.Id, dogs.Id)
+			}.ToJsonString());
+
+			var account = await _host.Get<IRemoteActorService>().GetActor(feed, refresh: true, token);
+			Assert.NotNull(account);
+			await _host.Run(j => j.Kind == JobKind.CountAccount && j.Payload == feed, token);
+
+			var shown = (await reader.Client.Get($"/api/v1/accounts/{account.ID}")).Ok().Body;
+			Assert.Equal(("Pets", 2), (shown.Text("display_name"), shown.Number("following_count")));
+			Assert.Contains("every pet", shown.Text("note"));
+			var communities = (await reader.Client.Get($"/api/v1/accounts/{account.ID}/following")).Ok().Array;
+			Assert.Equal(new[] { cats.Id, dogs.Id }, communities.Select(c => c.Text("uri")));
+			Assert.All(communities, c => Assert.True(c!["group"]!.GetValue<bool>()));
+			Assert.Equal(HttpStatusCode.UnprocessableEntity, (await reader.Client.Post($"/api/v1/accounts/{account.ID}/follow")).Status);
+		}
+	}
+}