vnassets/background: add bg removal via rembg + isnet-anime

rembg with ONNX Runtime on CPU was chosen over BiRefNet: the latter
requires deform_conv2d which crashes on ROCm bfloat16 and runs at 40s
in float32. rembg delivers 0.23s per 1024x1024 image, no GPU deps,
and isnet-anime is trained specifically on anime images — exactly the
target domain.

CLI and session API both support single-image and batch (plural-first)
modes, reusing one model session across files.

pyproject.toml

@@ -16,6 +16,8 @@ dependencies = [
     "pyyaml",
     "click",
     "compel",
+    "rembg",
+    "onnxruntime",
 ]
 
 [tool.setuptools.packages.find]