refactor(interfaces): restructure interfaces into model-specific subfolders

• Create dedicated interface subfolders for each model (user, node, workflow)
  • Move user interfaces to interfaces/user/interface.ts
  • Move node interfaces to interfaces/node/interface.ts
  • Move workflow interfaces to interfaces/workflow/interface.ts
  • Update all import paths across models, controllers, services, and validators
  • Remove old consolidated interface files
  • Establish consistent interfaces/{modelName}/interface.ts structure