perf(federation): always prefer the shared inbox

Followers-only posts and DMs went to each recipient's personal inbox,
so a followers-only post was sent once per follower on the same server.
Mastodon, GoToSocial and Iceshrimp.NET always use the shared inbox.
Only activities with bcc or bto still go to personal inboxes.

Based on Akkoma a80444041.

Co-authored-by: Oneric <oneric@oneric.stub>

test/pleroma/web/activity_pub/publisher_test.exs

@@ -137,7 +137,7 @@ defmodule Pleroma.Web.ActivityPub.PublisherTest do
       assert Publisher.determine_inbox(activity, user) == "http://example.com/inbox"
     end
 
-    test "it returns inbox for messages involving single recipients in total" do
+    test "it returns sharedInbox for messages to a single recipient" do
       user =
         insert(:user, %{
           shared_inbox: "http://example.com/inbox",
@@ -148,6 +148,46 @@ defmodule Pleroma.Web.ActivityPub.PublisherTest do
         data: %{"to" => [user.ap_id], "cc" => []}
       }
 
+      assert Publisher.determine_inbox(activity, user) == "http://example.com/inbox"
+    end
+
+    test "it returns sharedInbox for followers-only messages" do
+      user =
+        insert(:user, %{
+          shared_inbox: "http://example.com/inbox",
+          inbox: "http://example.com/personal-inbox"
+        })
+
+      author = insert(:user)
+
+      activity = %Activity{
+        data: %{"to" => [author.follower_address], "cc" => []}
+      }
+
+      assert Publisher.determine_inbox(activity, user) == "http://example.com/inbox"
+    end
+
+    test "it returns inbox for messages with bcc or bto, which needs personal inboxes" do
+      user =
+        insert(:user, %{
+          shared_inbox: "http://example.com/inbox",
+          inbox: "http://example.com/personal-inbox"
+        })
+
+      for field <- ["bcc", "bto"] do
+        activity = %Activity{
+          data: %{"to" => [@as_public], "cc" => [], field => [user.ap_id]}
+        }
+
+        assert Publisher.determine_inbox(activity, user) == "http://example.com/personal-inbox"
+      end
+    end
+
+    test "it returns inbox when the user has no sharedInbox" do
+      user = insert(:user, %{shared_inbox: nil, inbox: "http://example.com/personal-inbox"})
+
+      activity = %Activity{data: %{"to" => [@as_public], "cc" => []}}
+
       assert Publisher.determine_inbox(activity, user) == "http://example.com/personal-inbox"
     end
   end