Fix tests for renamed fixtures and new stacked notifications

Clear assignee's existing notifications in setup since Notifier now
uses create_or_find_by instead of create, and reload the association
to avoid caching after destroy_all.

Update fixture references after rebase: use `logo_assignment_kevin`
as base notification and `logo_mentioned_david` for mention tests.
Update source to `logo_published` in tests that need generic card events.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/models/notification/pushable.rb

@@ -4,8 +4,7 @@ module Notification::Pushable
   included do
     class_attribute :push_targets, default: []
 
-    after_create_commit :push_later
-    after_update_commit :push_later, if: :source_id_previously_changed?
+    after_save_commit :push_later, if: :source_id_previously_changed?
   end
 
   class_methods do