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_helpers.py

@@ -1301,29 +1301,42 @@ def _filter_footer_words(
     img_h: int,
     log: Any,
     session_id: str,
-) -> None:
+) -> Optional[Dict]:
     """Remove isolated words in the bottom 5% of the page (page numbers).
 
-    Modifies *words* in place.
+    Modifies *words* in place and returns a page_number metadata dict
+    if a page number was extracted, or None.
     """
     if not words or img_h <= 0:
-        return
+        return None
     footer_y = img_h * 0.95
     footer_words = [
         w for w in words
         if w["top"] + w.get("height", 0) / 2 > footer_y
     ]
     if not footer_words:
-        return
+        return None
     # Only remove if footer has very few words (≤ 3) with short text
     total_text = "".join((w.get("text") or "").strip() for w in footer_words)
     if len(footer_words) <= 3 and len(total_text) <= 10:
+        # Extract page number metadata before removing
+        page_number_info = {
+            "text": total_text.strip(),
+            "y_pct": round(footer_words[0]["top"] / img_h * 100, 1),
+        }
+        # Try to parse as integer
+        digits = "".join(c for c in total_text if c.isdigit())
+        if digits:
+            page_number_info["number"] = int(digits)
+
         footer_set = set(id(w) for w in footer_words)
         words[:] = [w for w in words if id(w) not in footer_set]
         log.info(
-            "build-grid session %s: removed %d footer words ('%s')",
-            session_id, len(footer_words), total_text,
+            "build-grid session %s: extracted page number '%s' and removed %d footer words",
+            session_id, total_text, len(footer_words),
         )
+        return page_number_info
+    return None
 
 
 def _filter_header_junk(