fix(whisperx): pin torch CPU variant to fix uv resolution failure

Pin torch==2.8.0+cpu so uv resolves the CPU wheel from the extra
index instead of picking torch==2.8.0+cu128 from PyPI, which pulls
unresolvable CUDA dependencies.

Signed-off-by: eureka928 <meobius123@gmail.com>

Author: eureka0928

backend/python/whisperx/requirements-cpu.txt

@@ -1,3 +1,3 @@
 --extra-index-url https://download.pytorch.org/whl/cpu
-torch
+torch==2.8.0+cpu
 whisperx @ git+https://github.com/m-bain/whisperX.git