fix(ocr): persist model cache across container restarts

Surya downloads models from HuggingFace to /root/.cache on first use.
Without a volume, every container restart re-downloads ~73MB+.
Added ocr_cache volume to persist the cache.

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

docker-compose.yml

@@ -82,6 +82,7 @@ services:
     memswap_limit: 8g
     volumes:
       - ocr_models:/app/models
+      - ocr_cache:/root/.cache
     environment:
       KRAKEN_MODEL_PATH: /app/models/german_kurrent.mlmodel
       OCR_CONFIDENCE_THRESHOLD: "0.3"
@@ -186,3 +187,4 @@ volumes:
   frontend_node_modules:
   maven_cache:
   ocr_models:
+  ocr_cache: