PieFed joins the pasture; the instance actor answers at the root

PieFed 1.7.17 (dockurr's image of the release) runs in the pasture with its
Celery worker on the shared Postgres and Redis, and scenarios/piefed.sh
checks it both ways: 29 checks, communities, titled threads, comments, votes
up and down, a community poll and a vote in it, private messages, a
moderator's lock, unlock and removal, the unfollow and statistics.

What it showed:
- PieFed sends a community's announces to the inbox of the Application at a
  peer's root (as Lemmy serves its site actor) and to /inbox otherwise.
  PrivaPub answered 404 at its root, so every announce went to an /inbox it
  does not have. The instance actor now answers at / for ActivityPub
  requests, unsigned under SecureMode as at its own address.
- PieFed keeps serving a thread its moderator removed, so the removal could
  never be checked against the post's origin. A community on the post's own
  server now speaks for it; one elsewhere still waits for the origin to say
  the post is gone.

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

PrivaPub.Tests/Federation/CommunityEditsTests.cs

@@ -30,11 +30,12 @@ namespace PrivaPub.Tests.Federation
 
 		static string Origin(RemoteActor actor) => new Uri(actor.Id).GetLeftPart(UriPartial.Authority);
 
-		async Task<(RemoteActor Community, RemoteActor Poster, JsonObject Note, Post Post)> Announced()
+		// a post in a community the persona follows; the community on the poster's server unless another origin is given
+		async Task<(RemoteActor Community, RemoteActor Poster, JsonObject Note, Post Post)> Announced(string communityOrigin = default)
 		{
 			var token = TestContext.Current.CancellationToken;
 			var (root, alice) = await _harness.Persona("alice");
-			var community = new RemoteActor(_harness.Peer, "cats", type: "Group");
+			var community = new RemoteActor(_harness.Peer, "cats", communityOrigin, type: "Group");
 			var poster = new RemoteActor(_harness.Peer, "poster");
 			await _harness.Follows.Follow(root, new FollowForm { AvatarId = alice.Id, Target = community.Id }, token);
 			await DB.Default.Update<Following>().Match(f => f.AvatarId == alice.Id).Modify(f => f.State, FollowState.Accepted).ExecuteAsync(token);
@@ -109,6 +110,43 @@ namespace PrivaPub.Tests.Federation
 			Assert.True(await DB.Default.Find<DeletedObject>().Match(d => d.ObjectURI == post.ObjectURI).ExecuteAnyAsync(token));
 		}
 
+		// PieFed keeps serving a thread its moderator removed: a community on the post's own server speaks for it
+		[Fact]
+		public async Task A_removal_a_community_on_the_posts_server_announces_is_taken_though_the_post_is_still_served()
+		{
+			var token = TestContext.Current.CancellationToken;
+			var (community, poster, note, post) = await Announced();
+
+			await Announce(community, new JsonObject
+			{
+				["id"] = $"{Origin(poster)}/delete/{Guid.NewGuid():N}", ["type"] = "Delete", ["actor"] = poster.Id,
+				["object"] = note["id"]!.GetValue<string>(), ["summary"] = "off topic"
+			});
+
+			Assert.False(await DB.Default.Find<Post>().Match(p => p.ID == post.ID).ExecuteAnyAsync(token));
+		}
+
+		[Fact]
+		public async Task A_removal_a_community_elsewhere_announces_waits_for_the_posts_server_to_say_it_is_gone()
+		{
+			var token = TestContext.Current.CancellationToken;
+			var (community, poster, note, post) = await Announced(_harness.Peer.B);
+			var delete = new JsonObject
+			{
+				["id"] = $"{Origin(community)}/delete/{Guid.NewGuid():N}", ["type"] = "Delete", ["actor"] = community.Id,
+				["object"] = note["id"]!.GetValue<string>()
+			};
+
+			await Announce(community, (JsonObject)delete.DeepClone());
+			var kept = await DB.Default.Find<Post>().Match(p => p.ID == post.ID).ExecuteAnyAsync(token);
+			_harness.Peer.Serve(new Uri(note["id"]!.GetValue<string>()).AbsolutePath, new JsonObject { ["id"] = note["id"]!.GetValue<string>(), ["type"] = "Tombstone" }.ToJsonString());
+			delete["id"] = $"{Origin(community)}/delete/{Guid.NewGuid():N}";
+			await Announce(community, delete);
+
+			Assert.True(kept);
+			Assert.False(await DB.Default.Find<Post>().Match(p => p.ID == post.ID).ExecuteAnyAsync(token));
+		}
+
 		[Fact]
 		public async Task A_fetched_object_does_not_wear_the_signature_of_the_activity_that_caused_the_fetch()
 		{