Implement multiple audio/image model support with aliases

- Add support for multiple --audio-model arguments (action='append')
- Add support for multiple --image-model arguments (action='append')
- Add 'audio' alias pointing to first audio model
- Add 'vision'/'image' aliases pointing to first image model
- Update MultiModelManager to store audio_models and image_models as lists
- Add audio_model and image_model properties for accessing first model
- Update get_model_for_request to handle aliases
- Update list_models to show all models and aliases
- Fix remaining references in main function to use list-based variables