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/engines/ipa_repair.py

@@ -14,7 +14,7 @@ Contains:
 
 import logging
 import re
-from typing import Any, Dict, List, Optional
+from typing import List
 
 from ..types import IPA_AVAILABLE
 from .ipa_lookup import (