Add Ground Truth regression test system for OCR pipeline

Extract _build_grid_core() from build_grid() endpoint for reuse.
New ocr_pipeline_regression.py with endpoints to mark sessions as
ground truth, list them, and run regression comparisons after code
changes. Frontend button in StepGroundTruth.tsx to mark/update GT.

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

klausur-service/backend/ocr_pipeline_api.py

@@ -47,6 +47,7 @@ from ocr_pipeline_ocr_merge import (
 )
 from ocr_pipeline_postprocess import router as _postprocess_router
 from ocr_pipeline_auto import router as _auto_router
+from ocr_pipeline_regression import router as _regression_router
 
 # ---------------------------------------------------------------------------
 # Composite router (used by main.py)
@@ -59,3 +60,4 @@ router.include_router(_words_router)
 router.include_router(_ocr_merge_router)
 router.include_router(_postprocess_router)
 router.include_router(_auto_router)
+router.include_router(_regression_router)