fix(ocr-pipeline): prevent grid from producing more rows than gap-based

Two fixes:
1. Grid validation: reject word-center grid if it produces MORE rows
   than gap-based detection (more rows = lines were split = worse).
   Falls back to gap-based rows in that case.

2. Words overlay: draw clean grid cells (column × row intersections)
   instead of padded entry bboxes. Eliminates confusing double lines.
   OCR text labels are placed inside the grid cells directly.

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

klausur-service/backend/cv_vocab_pipeline.py

@@ -1829,6 +1829,13 @@ def _regularize_row_grid(
     # Remove empty grid rows (no words assigned)
     grid_rows = [gr for gr in grid_rows if gr.word_count > 0]
 
+    # The grid must not produce MORE rows than gap-based detection.
+    # More rows means the clustering split actual lines — that's worse.
+    if len(grid_rows) > len(content_rows):
+        logger.info(f"RowGrid: grid produced {len(grid_rows)} rows > "
+                    f"{len(content_rows)} gap-based → keeping gap-based rows")
+        return rows
+
     # --- Step H: Merge header/footer + re-index ---
     result = list(non_content) + grid_rows
     result.sort(key=lambda r: r.y)