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>

docker-compose.yml

@@ -87,9 +87,11 @@ services:
     memswap_limit: 12g
     volumes:
       - ocr_models:/app/models
-      - ocr_cache:/app/cache
+      - ocr_cache:/app/cache  # HuggingFace / ketos cache — prevents re-downloads on recreate (HF_HOME)
     environment:
       HF_HOME: /app/cache
+      XDG_CACHE_HOME: /app/cache
+      TORCH_HOME: /app/models/torch
       KRAKEN_MODEL_PATH: /app/models/german_kurrent.mlmodel
       TRAINING_TOKEN: "${OCR_TRAINING_TOKEN:-}"
       OCR_CONFIDENCE_THRESHOLD: "0.3"