Idempotency guard

Ensure a task can’t be double-picked by two processes. Use `FOR UPDATE SKIP LOCKED` or atomic status transition.

Look for the task picking logic, likely in a service or worker file. Research PostgreSQL's FOR UPDATE SKIP LOCKED for row-level locking and implement an atomic status transition from 'pending' to 'processing'. Check existing database schema for the task status field and ensure the update query is idempotent.

索引モデルが issue の本文から書いたものです。