security(ocr): redirect XDG cache and Torch home away from read-only HOME

Prevents PyTorch/Matplotlib/Ketos from writing to /home/ocr which is
on the read-only container filesystem — fixes Nora's blocker. Also
restores the explanatory comment on the ocr_cache volume mount.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ocr-service/Dockerfile

@@ -30,6 +30,8 @@ RUN chmod +x /app/entrypoint.sh
 
 ENV HOME=/home/ocr
 ENV HF_HOME=/app/cache
+ENV XDG_CACHE_HOME=/app/cache
+ENV TORCH_HOME=/app/models/torch
 
 USER ocr