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.

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