pin av==13.1.0 (14.4.0 has no manylinux wheel)

requirements.txt

@@ -17,7 +17,10 @@ accelerate>=0.34
 safetensors
 
 # torchvision.io.write_video is a thin PyAV wrapper; torchvision does not declare it.
-av>=13.0,<15
+# Pinned exactly: av 14.4.0 publishes no manylinux x86_64 wheel and falls back to a
+# source build that needs FFmpeg dev headers; 13.1.0 is the repo's declared floor and
+# ships a cp312 wheel.
+av==13.1.0
 numpy<3
 pillow
 requests