#454 Fixed flaky durable_cron integration tests

Tests were failing intermittently due to internal queue backlog from parallel test execution. **Root causes:** 1. TestDurableCronBasicFunctionality missing ensure_queue_capacity fixture 2. Short timeouts (15-30s) not enough for busy queues 3. Only 1 internal worker overwhelmed by 8 parallel test workers **Fixes:** - Added ensure_queue_capacity fixture to TestDurableCronAtomicity and TestDurableCronBasicFunctionality - Increased task execution timeouts from 60s to 120s - Scaled internal-worker from 1 to 2 replicas in docker-compose.yml **Result:** All 183 integration tests pass (0 failures)