feat(workflow): implement workflow execution with OpenAI integration

• Create WorkflowService with executeWorkflow method for sequential node processing
  • Add workflow execution controller action
  • Implement POST /api/workflow/run endpoint for workflow execution
  • Add entry node detection logic
  • Implement prompt template with {{input}} and {{lastOutput}} placeholders
  • Add comprehensive error handling for OpenAI API calls
  • Support sequential workflow execution through node chains