fix: move column expansion AFTER sub-column split

The narrow column expansion was running inside detect_column_geometry()
on the 4 main columns, but the narrowest columns (marker ~14px,
page_ref ~93px) are created AFTERWARDS by _detect_sub_columns().

Extracted expand_narrow_columns() as standalone function and call it
after sub-column splitting in the columns API endpoint.

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

klausur-service/backend/ocr_pipeline_api.py

@@ -51,6 +51,7 @@ from cv_vocab_pipeline import (
     deskew_image_by_word_alignment,
     detect_column_geometry,
     detect_row_geometry,
+    expand_narrow_columns,
     _apply_shear,
     dewarp_image,
     dewarp_image_manual,
@@ -802,6 +803,9 @@ async def detect_columns(session_id: str):
         geometries = _detect_sub_columns(geometries, content_w, left_x=left_x,
                                           top_y=top_y, header_y=header_y, footer_y=footer_y)
 
+        # Expand narrow columns (sub-columns are often very narrow)
+        geometries = expand_narrow_columns(geometries, content_w, left_x, word_dicts)
+
         # Phase B: Content-based classification
         regions = classify_column_types(geometries, content_w, top_y, w, h, bottom_y,
                                         left_x=left_x, right_x=right_x, inv=inv)