#760 Fix function-level imports of banned modules causing sandbox RestrictedImportError

Issue #760: Function-level imports in engine code (config.py, workflow_executor.py, apps/sdk/context.py, rbac_manager.py) caused RestrictedImportError when sandbox was active. Fixed by moving imports to module level. Files modified: engine/config.py (lines 830, 853), engine/tools/workflow_executor.py (line 335), engine/apps/sdk/context.py (line 652), engine/security/rbac_manager.py (line 94).