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.prod.yml

@@ -38,13 +38,17 @@ services:
       STRIPE_ENTERPRISE_MONTHLY_PRICE_ID: prod_Toj8xGEui9jl6j
       STRIPE_ENTERPRISE_YEARLY_PRICE_ID: prod_Toj9A7A773xrdn
 
-  # OCR - Production log level
+  # OCR - Production log level + engine config
   mvp-ocr:
     environment:
       LOG_LEVEL: error
       REDIS_HOST: mvp-redis
       REDIS_PORT: 6379
       REDIS_DB: 1
+      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
 
   # PostgreSQL - Remove dev ports, production log level
   mvp-postgres: