fix(ocr-pipeline): preserve sub-column data in vocab table display

Three fixes for sub-columns disappearing at end of streaming:

1. Backend: add column_marker mapping in _cells_to_vocab_entries()
   so marker text is included in vocab entries (not silently dropped)

2. Frontend types: add source_page and bbox_ref to WordEntry interface

3. Frontend table: show page_ref column (Seite) in vocab table when
   entries have source_page data, instead of only EN/DE/Example

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

klausur-service/backend/cv_vocab_pipeline.py

@@ -4215,12 +4215,14 @@ def _cells_to_vocab_entries(
         'column_de': 'german',
         'column_example': 'example',
         'page_ref': 'source_page',
+        'column_marker': 'marker',
     }
     bbox_key_map = {
         'column_en': 'bbox_en',
         'column_de': 'bbox_de',
         'column_example': 'bbox_ex',
         'page_ref': 'bbox_ref',
+        'column_marker': 'bbox_marker',
     }
 
     # Group cells by row_index
@@ -4238,12 +4240,14 @@ def _cells_to_vocab_entries(
             'german': '',
             'example': '',
             'source_page': '',
+            'marker': '',
             'confidence': 0.0,
             'bbox': None,
             'bbox_en': None,
             'bbox_de': None,
             'bbox_ex': None,
             'bbox_ref': None,
+            'bbox_marker': None,
             'ocr_engine': row_cells[0].get('ocr_engine', '') if row_cells else '',
         }