Do not notify users unable to access post

Known implementations all intend to only tag users who are _also_ allowed to access the post,
but this is not guaranteed by ActivityPub semantics, thus recheck against the authorative
AP addressing fields.
An alternative worth considering is directly using the addressing fields to decide whom
to notify ignoring tags. However, MastoAPI later uses tags and thus this may lead to
confusing results if a recipient is addressed but not tagged and receives a notification about
being mentioned while the API claims they are not mentioned.

Addresses the receiving side of: https://akkoma.dev/AkkomaGang/akkoma/issues/1135

(cherry picked from commit 5cff1fac24bb3e6c22160202a7b846f8a62f2477)

lib/pleroma/web/common_api/utils.ex

@@ -435,13 +435,54 @@ defmodule Pleroma.Web.CommonAPI.Utils do
           %{}
       end
 
-    tagged_mentions = maybe_extract_mentions(object_data)
+    tagged_mentions =
+      maybe_extract_mentions(object_data)
+      |> filter_tags_to_qualified(activity)
 
     recipients ++ tagged_mentions
   end
 
   def maybe_notify_mentioned_recipients(recipients, _), do: recipients
 
+  # Filter tagged mentions to ensure they can actually access the status
+  defp filter_tags_to_qualified(tagged_mentions, %Activity{
+         recipients: recipients,
+         data: data,
+         actor: actor
+       }) do
+    recipients =
+      if recipients && recipients != [],
+        do: recipients,
+        else:
+          (data["to"] || []) ++ (data["cc"] || []) ++ (data["bto"] || []) ++ (data["bcc"] || [])
+
+    cond do
+      recipients == [] ->
+        []
+
+      Pleroma.Constants.as_public() in recipients ->
+        tagged_mentions
+
+      true ->
+        author = User.get_cached_by_ap_id(actor)
+        to_followers = author.follower_address in recipients
+
+        Enum.filter(tagged_mentions, fn tagged ->
+          cond do
+            tagged in recipients ->
+              true
+
+            to_followers ->
+              tagged_user = User.get_cached_by_ap_id(tagged)
+              User.following?(tagged_user, author)
+
+            true ->
+              false
+          end
+        end)
+    end
+  end
+
   def maybe_notify_followers(recipients, %Activity{data: %{"type" => "Move"}} = activity) do
     with %User{} = user <- User.get_cached_by_ap_id(activity.actor) do
       user