chore: update Docker and compose files for PaddleOCR engine (refs #119)

- Replace libtesseract-dev with libgomp1 (OpenMP for PaddlePaddle)
- Pre-download PP-OCRv4 models during Docker build
- Add OCR engine env vars to all compose files (base, staging, prod)
- Add optional Google Vision secret mount (commented, enable on demand)
- Create google-vision-key.json.example placeholder

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

docker-compose.yml

@@ -193,8 +193,16 @@ services:
       REDIS_HOST: mvp-redis
       REDIS_PORT: 6379
       REDIS_DB: 1
+      # OCR engine configuration (PaddleOCR primary, cloud fallback optional)
+      OCR_PRIMARY_ENGINE: paddleocr
+      OCR_FALLBACK_ENGINE: ${OCR_FALLBACK_ENGINE:-none}
+      OCR_FALLBACK_THRESHOLD: ${OCR_FALLBACK_THRESHOLD:-0.6}
+      GOOGLE_VISION_KEY_PATH: /run/secrets/google-vision-key.json
     volumes:
       - /tmp/vin-debug:/tmp/vin-debug
+      # Optional: Uncomment to enable Google Vision cloud fallback.
+      # Requires: secrets/app/google-vision-key.json and OCR_FALLBACK_ENGINE=google_vision
+      # - ./secrets/app/google-vision-key.json:/run/secrets/google-vision-key.json:ro
     networks:
       - backend
       - database