feat: add optional Google Vision cloud fallback engine (refs #118)

CloudEngine wraps Google Vision TEXT_DETECTION with lazy init.
HybridEngine runs primary engine, falls back to cloud when confidence
is below threshold. Disabled by default (OCR_FALLBACK_ENGINE=none).

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

ocr/app/config.py

@@ -17,6 +17,15 @@ class Settings:
             os.getenv("OCR_CONFIDENCE_THRESHOLD", "0.6")
         )
 
+        # Cloud fallback configuration (disabled by default)
+        self.ocr_fallback_engine: str = os.getenv("OCR_FALLBACK_ENGINE", "none")
+        self.ocr_fallback_threshold: float = float(
+            os.getenv("OCR_FALLBACK_THRESHOLD", "0.6")
+        )
+        self.google_vision_key_path: str = os.getenv(
+            "GOOGLE_VISION_KEY_PATH", "/run/secrets/google-vision-key.json"
+        )
+
         # Redis configuration for job queue
         self.redis_host: str = os.getenv("REDIS_HOST", "mvp-redis")
         self.redis_port: int = int(os.getenv("REDIS_PORT", "6379"))