Fix data insertion in NewThreadFollower migration

Author: trichoplax

db/migrate/20251203164131_create_new_thread_followers.rb

@@ -17,7 +17,10 @@ def create_table_new_thread_followers
 
   def move_rows_with_non_nil_post_id
     NewThreadFollower.insert_all(
-      ThreadFollower.select(:user_id, :post_id, :created_at, :updated_at).where.not(post_id:nil)
+      ThreadFollower.select(:user_id, :post_id, :created_at, :updated_at)
+        .where.not(post_id:nil)
+        .to_a
+        .map(&:attributes)
     )
     ThreadFollower.where.not(post_id:nil).delete_all
   end