Fix uv index configuration: Use PyTorch CUDA as additional index

- Changed from 'default' to named additional index
- Added tool.uv.sources to specify torch comes from pytorch-cu121 index
- Other packages (fastapi, uvicorn, etc.) still come from PyPI
- Fixes: 'fastapi was not found in the package registry' error

How it works:
- PyPI remains the default index for most packages
- torch package explicitly uses pytorch-cu121 index
- Best of both worlds: CUDA PyTorch + all other packages from PyPI

pyproject.toml

@@ -57,11 +57,15 @@ dev = [
     "pyinstaller>=6.17.0",
 ]
 
-# Use PyTorch CUDA index by default
+# Add PyTorch CUDA index as additional source
 # CUDA builds work on both GPU and CPU systems (fallback to CPU if no GPU)
 [[tool.uv.index]]
+name = "pytorch-cu121"
 url = "https://download.pytorch.org/whl/cu121"
-default = true
+
+# Tell uv to get torch from the PyTorch CUDA index
+[tool.uv.sources]
+torch = { index = "pytorch-cu121" }
 
 [tool.ruff]
 line-length = 100