Fix: Resolve all lint errors from ocr/ restructure

- Added ocr_region import to cell_grid/build.py and legacy.py
- Fixed circular import in engines.py via lazy import
- Auto-fixed 22 unused imports via ruff --fix

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

klausur-service/backend/ocr/preprocessing/preprocessing.py

@@ -17,10 +17,6 @@ from typing import Tuple
 
 import numpy as np
 
-from ..types import (
-    CV2_AVAILABLE,
-    TESSERACT_AVAILABLE,
-)
 
 logger = logging.getLogger(__name__)