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/cell_grid/legacy.py

@@ -13,6 +13,7 @@ from typing import Any, Dict, List, Optional, Tuple
 import numpy as np
 
 from ..types import PageRegion, RowGeometry
+from ..review.pipeline import ocr_region  # noqa: F401 — Tesseract wrapper
 from ..engines.engines import (
     RAPIDOCR_AVAILABLE,
     _assign_row_words_to_columns,