Remove pinned numpy and scikit-image to fix rembg dependency resolution

rembg>=2.0.70 requires scikit-image>=0.26.0, which conflicted with
the pinned scikit-image==0.22.0. Instead of pinning specific versions,
let pip resolve compatible versions automatically based on rembg's
requirements.

Also removed numpy pin as it may conflict with torch/rembg dependencies
- pip will select a compatible version.

https://claude.ai/code/session_01MYpjNQXD1fZE4gCweGU4QQ

backend/requirements.txt

@@ -2,7 +2,7 @@ fastapi==0.109.0
 uvicorn[standard]==0.27.0
 python-multipart==0.0.6
 Pillow>=12.1.0,<13.0.0
-numpy==2.3.0
+# numpy version will be resolved by pip based on torch/rembg requirements
 sqlalchemy==2.0.25
 python-jose[cryptography]==3.3.0
 passlib[bcrypt]==1.7.4
@@ -13,7 +13,7 @@ pydantic==2.5.3
 pydantic-settings==2.1.0
 email-validator==2.1.0
 opencv-python-headless==4.9.0.80
-scikit-image==0.22.0
+# scikit-image removed - let pip resolve it automatically via rembg dependency
 # rembg for background removal with BiRefNet models (state-of-the-art)
 # Using latest onnxruntime (1.17+ fixed executable stack issues)
 onnxruntime>=1.17.0