Extract page number as metadata instead of silently removing it

_filter_footer_words now returns page number info (text, y_pct, number)
instead of just removing footer words. The page number is included in
the grid result as `page_number` and displayed in the frontend summary
bar as "S. 233".

This preserves page numbers for later page concatenation in the
customer frontend while still removing them from the grid content.

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

klausur-service/backend/grid_editor_api.py

@@ -124,8 +124,9 @@ async def _build_grid_core(
 
     # 2c. Filter footer rows (page numbers at the very bottom).
     # Isolated short text in the bottom 5% of the page is typically a
-    # page number ("64", "S. 12") and not real content.
-    _filter_footer_words(all_words, img_h, logger, session_id)
+    # page number ("64", "S. 12") and not real content.  The page number
+    # is extracted as metadata for the frontend header display.
+    page_number_info = _filter_footer_words(all_words, img_h, logger, session_id)
 
     # 2c2. Filter OCR junk from header illustrations.
     # Low-confidence short fragments above the first real content row.
@@ -1668,6 +1669,7 @@ async def _build_grid_core(
             "ipa_applied": bool(ipa_target_cols) if not skip_ipa else False,
             "syllables_applied": syllable_insertions > 0,
         },
+        "page_number": page_number_info,
         "duration_seconds": round(duration, 2),
     }