fix(ocr-pipeline): pass left_x/right_x to classify_column_types in API path

The ocr_pipeline_api.py code path called classify_column_types without
left_x/right_x, so margin regions were never created. Also add logging
to _build_margin_regions for debugging.

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

klausur-service/backend/cv_vocab_pipeline.py

@@ -2062,6 +2062,10 @@ def _build_margin_regions(
             classification_method='content_bounds',
         ))
 
+    if margins:
+        logger.info(f"Margins: {[(m.type, m.x, m.width) for m in margins]} "
+                     f"(left_x={left_x}, right_x={right_x}, img_w={img_w})")
+
     return margins