#455 HIGH: 9 singleton race conditions without locking

Multiple singletons in engine/ lack thread-safe initialization: 1. AbsurdClient (absurd_client.py:1460-1473) 2. AppExecutor (apps/executor.py:943-958) 3. DatabaseCodeLoader (apps/code_loader.py:540-555) 4. AppToolBridge (apps/tool_bridge.py:340-355) 5. DeadLetterService (services/dead_letter_service.py:387-392) 6. HeartbeatService (services/heartbeat_service.py:237-242) 7. StorageService (services/storage_service.py:699-702) 8. S3StorageProvider (storage/s3_provider.py:428-431) 9. ClientKeyManager (security/rbac_manager.py:396-402) Fix: Add double-check locking with threading.Lock() like tools/registry.py pattern.