Add local SAM model support for offline Smart Select

- Add torch, torchvision, segment-anything to requirements
- Create download_sam_model.py script to fetch SAM checkpoint
- Update tools.py to use local SAM with Replicate API fallback
- Add SAM model check to entrypoint.sh with helpful instructions
- Model persists in /app/data/models via Docker volume mount

backend/requirements.txt

@@ -16,3 +16,7 @@ opencv-python-headless==4.9.0.80
 scikit-image==0.22.0
 rembg==2.0.50
 onnxruntime==1.16.3
+# SAM (Segment Anything) for smart object selection - runs locally, no API needed
+torch==2.1.2
+torchvision==0.16.2
+segment-anything @ git+https://github.com/facebookresearch/segment-anything.git