fix(streaming): include page_ref columns in SSE metadata

The streaming word endpoint excluded page_ref from _skip_types,
causing sub-column splits to be lost in the meta event and final
grid_shape. Aligned _skip_types with build_cell_grid_streaming().

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

klausur-service/backend/ocr_pipeline_api.py

@@ -1237,7 +1237,7 @@ async def _word_stream_generator(
 
     # Compute grid shape upfront for the meta event
     n_content_rows = len([r for r in row_geoms if r.row_type == 'content'])
-    _skip_types = {'column_ignore', 'header', 'footer', 'margin_top', 'margin_bottom', 'page_ref'}
+    _skip_types = {'column_ignore', 'header', 'footer', 'margin_top', 'margin_bottom', 'margin_left', 'margin_right'}
     n_cols = len([c for c in col_regions if c.type not in _skip_types])
 
     # Determine layout