Integrate Cloudflare R2 for CUDA binary hosting

- Update CI workflow to upload CUDA binary to R2 instead of GitHub
- Use custom domain: downloads.voicebox.sh
- Update release notes with R2 download link
- Add R2 setup documentation with GitHub secrets instructions
- Add local test script for R2 uploads
- Fixes GitHub 2GB release asset limit issue

Required GitHub secrets:
- R2_ACCESS_KEY_ID
- R2_SECRET_ACCESS_KEY
- R2_ENDPOINT

Cost: ~$0.04/month (free bandwidth with R2)

backend/test_r2_upload.sh

@@ -0,0 +1,86 @@
+#!/bin/bash
+# Test R2 upload locally before running in CI
+
+set -e
+
+echo "============================================================"
+echo "Cloudflare R2 Upload Test"
+echo "============================================================"
+
+# Check for required environment variables
+if [ -z "$AWS_ACCESS_KEY_ID" ] || [ -z "$AWS_SECRET_ACCESS_KEY" ] || [ -z "$R2_ENDPOINT" ]; then
+    echo "ERROR: Missing required environment variables"
+    echo ""
+    echo "Please set:"
+    echo "  export AWS_ACCESS_KEY_ID='your-r2-access-key-id'"
+    echo "  export AWS_SECRET_ACCESS_KEY='your-r2-secret-access-key'"
+    echo "  export R2_ENDPOINT='https://your-account-id.r2.cloudflarestorage.com'"
+    echo ""
+    exit 1
+fi
+
+# Check for AWS CLI
+if ! command -v aws &> /dev/null; then
+    echo "Installing AWS CLI..."
+    pip install awscli
+fi
+
+# Find CUDA binary
+CUDA_BINARY=$(ls dist/voicebox-server-cuda*.exe 2>/dev/null | head -1)
+
+if [ -z "$CUDA_BINARY" ]; then
+    echo "ERROR: CUDA binary not found in dist/"
+    echo "Run: bash build_cuda.bat"
+    exit 1
+fi
+
+echo ""
+echo "Found CUDA binary: $CUDA_BINARY"
+echo "Size: $(du -h "$CUDA_BINARY" | cut -f1)"
+echo ""
+
+# Test version
+VERSION="v0.1.12-test"
+PLATFORM="x86_64-pc-windows-msvc"
+FILENAME="voicebox-server-cuda-${PLATFORM}.exe"
+
+echo "Test upload configuration:"
+echo "  Version: $VERSION"
+echo "  Platform: $PLATFORM"
+echo "  Endpoint: $R2_ENDPOINT"
+echo "  Bucket: voicebox-releases"
+echo "  Path: cuda/$VERSION/$FILENAME"
+echo ""
+
+read -p "Proceed with upload? (y/n) " -n 1 -r
+echo
+if [[ ! $REPLY =~ ^[Yy]$ ]]; then
+    echo "Aborted."
+    exit 1
+fi
+
+echo ""
+echo "Uploading to R2..."
+
+aws s3 cp "$CUDA_BINARY" \
+    "s3://voicebox-releases/cuda/${VERSION}/${FILENAME}" \
+    --endpoint-url "$R2_ENDPOINT" \
+    --acl public-read
+
+if [ $? -eq 0 ]; then
+    echo ""
+    echo "============================================================"
+    echo "Upload successful!"
+    echo "============================================================"
+    echo ""
+    echo "Download URL:"
+    echo "https://downloads.voicebox.sh/cuda/${VERSION}/${FILENAME}"
+    echo ""
+    echo "Test with:"
+    echo "curl -I https://downloads.voicebox.sh/cuda/${VERSION}/${FILENAME}"
+    echo ""
+else
+    echo ""
+    echo "Upload failed!"
+    exit 1
+fi