#472 Race Condition: StorageService.provider lazy init without lock

engine/services/storage_service.py:67-76 StorageService.provider property uses lazy initialization without double-check locking: ```python @property def provider(self): if self._provider is None: # CHECK self._provider = get_storage_provider() # USE - RACE return self._provider ``` Race: Two threads can check None simultaneously, both create provider instances. Impact: Duplicate resources, potential connection pool exhaustion. FIX: Add threading.Lock with double-check locking pattern.