#475 Race Condition: File deduplication missing ON CONFLICT

engine/services/storage_service.py:114-131 File store deduplication has TOCTOU race: 1. Store file in S3 2. Check if storage_ref exists in DB 3. Insert if not exists Race: Two threads store same file, both check DB, both insert -> duplicate key error. FIX: Use INSERT ... ON CONFLICT (storage_ref) DO NOTHING RETURNING file_id