Fix NameError: _text_has_garbled_ipa not imported in grid_editor_helpers

After refactoring grid_editor_api.py into helpers, the function
_text_has_garbled_ipa was used in _detect_heading_rows_by_single_cell
but never imported from cv_ocr_engines. This caused HTTP 500 on
build-grid for sessions that trigger single-cell heading detection.

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

klausur-service/backend/grid_editor_helpers.py

@@ -16,6 +16,7 @@ import cv2
 import numpy as np
 
 from cv_words_first import _cluster_rows, _build_cells
+from cv_ocr_engines import _text_has_garbled_ipa
 
 logger = logging.getLogger(__name__)