- Added unload_all_models() to MultiModelManager that handles ALL model types:
  ModelManager, diffusers pipelines, sd.cpp StableDiffusion, and any other objects
- Text endpoints now properly unload image models before loading text models
- Image endpoints now properly unload text models before loading image models
- The rule: in ondemand mode, if the model in VRAM differs from the requested
  model (regardless of type), fully unload before loading the new one
- Includes gc.collect(), torch.cuda.empty_cache(), and 1s settle delay