#691 VAL-04: DLQ not integrated with retry exhaustion

Location: timeout_service.py, dead_letter_service.py. Issue: TimeoutService marks tasks as failed but does NOT move them to DLQ. DeadLetterService exists but is never called. Tasks exceeding max_attempts remain in runs table forever. Fix: Call dead_letter_service.move_to_dlq() after max_attempts exhaustion.