Fix Pillow version to match rembg>=2.0.70 requirements

rembg 2.0.70+ requires Pillow>=12.1.0,<13.0.0 which conflicted with
the pinned Pillow==10.2.0. Updated to use the version range that
satisfies rembg while remaining compatible with scikit-image and
torchvision (both have no upper bound on Pillow).

https://claude.ai/code/session_01MYpjNQXD1fZE4gCweGU4QQ

backend/requirements.txt

@@ -1,7 +1,7 @@
 fastapi==0.109.0
 uvicorn[standard]==0.27.0
 python-multipart==0.0.6
-Pillow==10.2.0
+Pillow>=12.1.0,<13.0.0
 numpy==2.3.0
 sqlalchemy==2.0.25
 python-jose[cryptography]==3.3.0