fix(depth-estimation): add huggingface_hub as explicit dependency

Was only a transitive dep via gradio/depth-anything-v2, getting dropped
by pip's resolver. Now explicitly required for hf_hub_download.

Author: solderzzc

skills/transformation/depth-estimation/requirements.txt

@@ -4,6 +4,7 @@
 torch~=2.7.0
 torchvision~=0.22.0
 depth-anything-v2>=0.1.0
+huggingface_hub>=0.20.0
 numpy>=1.24.0
 opencv-python-headless>=4.8.0
 Pillow>=10.0.0