perf: cap torch threads per camera worker

Default TORCH_NUM_THREADS=1 so 14-20 workers do not each spawn a
28-thread BLAS pool.

tests/test_config.py

@@ -9,3 +9,7 @@ def test_parse_cameras_list():
 
 def test_default_beamform_mode_auto():
     assert Config().beamform_mode == "auto"
+
+
+def test_torch_num_threads_default_one():
+    assert Config().torch_num_threads == 1