Fix subprocess import error in detect_gpu_backends

- Move subprocess import to function scope to avoid UnboundLocalError
- Ensure subprocess is available for fallback GPU detection

vidai/compat.py

@@ -249,6 +249,8 @@ def get_platform_info() -> dict:
 
 def detect_gpu_backends() -> dict:
     """Detect available GPU backends (CUDA and ROCm)."""
+    import subprocess
+
     backends = {
         'cuda': False,
         'rocm': False,