Fix duplicate push notification for mention and comment event

Event notifiers used the `mentionees` DB association to exclude mentioned
users from comment/card notifications. Since mentions are created async
via Mention::CreateJob, a race condition meant the mentionee list could
be empty when the event notification job ran first, causing the user to
receive both a comment and a mention push notification.

Use `scan_mentionees` instead, which scans the rich text body directly
for mentioned users without depending on Mention records existing yet.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/models/notifier/card_event_notifier.rb

@@ -8,9 +8,9 @@ class Notifier::CardEventNotifier < Notifier
       when "card_assigned"
         source.assignees.excluding(creator)
       when "card_published"
-        board.watchers.without(creator, *card.mentionees).including(*card.assignees).uniq
+        board.watchers.without(creator, *card.scan_mentionees).including(*card.assignees).uniq
       when "comment_created"
-        card.watchers.without(creator, *source.eventable.mentionees)
+        card.watchers.without(creator, *source.eventable.scan_mentionees)
       else
         board.watchers.without(creator)
       end

app/models/notifier/comment_event_notifier.rb

@@ -3,7 +3,7 @@ class Notifier::CommentEventNotifier < Notifier
 
   private
     def recipients
-      card.watchers.without(creator, *source.eventable.mentionees)
+      card.watchers.without(creator, *source.eventable.scan_mentionees)
     end
 
     def card