Seed script

Add a script to insert sample tasks due in the next few minutes for manual testing.

Look for existing database seed or migration scripts in the project. The script should insert sample tasks with due dates set to the near future. Check the task model/schema to understand required fields. Run the script to verify tasks are inserted into the PostgreSQL database, then manually test the scheduler to see if they execute.

索引モデルが issue の本文から書いたものです。