Handle Undo Follow activities

lib/clacks/inbox.ex

@@ -123,6 +123,33 @@ defmodule Clacks.Inbox do
     end
   end
 
+  def handle(
+        %{
+          "type" => "Undo",
+          "actor" => follower_id,
+          "object" => %{"type" => "Follow", "object" => followee_id}
+        } = activity
+      )
+      when is_binary(followee_id) do
+    followee = Actor.get_by_ap_id(followee_id)
+
+    store_activity(activity)
+
+    changeset =
+      Actor.changeset(followee, %{
+        followers: List.delete(followee.followers, follower_id)
+      })
+
+    case Repo.update(changeset) do
+      {:error, changeset} ->
+        Logger.error("Couldn't store updated followers: #{inspect(changeset)}")
+        {:error, "Couldn't store updated followers"}
+
+      {:ok, _followee} ->
+        :ok
+    end
+  end
+
   # as a fallback, just store the activity
   def handle(activity) do
     Logger.debug("Unhandled activity: #{inspect(activity)}")