Fix false positive: exclude first/last rows from single-cell heading detection

Page numbers like "two hundred and twelve" in the last row were falsely
detected as headings. Now first and last non-header rows are excluded.

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

klausur-service/backend/grid_editor_api.py

@@ -706,11 +706,21 @@ def _detect_heading_rows_by_single_cell(
         if multi_col_rows < len(rows) * 0.4:
             continue
 
+        # Exclude first and last non-header rows — these are typically
+        # page numbers or footer text, not headings.
+        non_header_rows = [r for r in rows if not r.get("is_header")]
+        if len(non_header_rows) < 3:
+            continue
+        first_ri = non_header_rows[0]["index"]
+        last_ri = non_header_rows[-1]["index"]
+
         heading_row_indices = []
         for row in rows:
             if row.get("is_header"):
                 continue
             ri = row["index"]
+            if ri == first_ri or ri == last_ri:
+                continue
             row_cells = [c for c in cells if c.get("row_index") == ri]
             content_cells = [
                 c for c in row_cells

klausur-service/backend/tests/test_grid_editor_api.py

@@ -675,3 +675,22 @@ class TestDetectHeadingRowsBySingleCell:
         assert len(heading) == 1
         assert heading[0]["col_type"] == "heading"
         assert heading[0]["col_index"] == 1  # Should start at column_2, not 0
+
+    def test_last_row_single_cell_not_heading(self):
+        """Single-cell in last row (e.g. page number '212') → NOT heading."""
+        zone = self._make_vocab_zone()
+        # Make row 7 (the last) have only 1 cell in column_2
+        zone["cells"] = [c for c in zone["cells"] if c["row_index"] != 7]
+        zone["cells"].append({
+            "cell_id": "Z0_R07_C1",
+            "zone_index": 0, "row_index": 7, "col_index": 1,
+            "col_type": "column_2", "text": "two hundred and twelve",
+            "word_boxes": [self._make_word_box("two", 130, 310, 30, 20)],
+        })
+        zones_data = [zone]
+        count = _detect_heading_rows_by_single_cell(zones_data, 800, 1000)
+        # Row 4 "Theme" = heading, but row 7 (last) should NOT be heading
+        assert count == 1
+        heading_cells = [c for c in zone["cells"]
+                         if c.get("col_type") == "heading"]
+        assert all(c["row_index"] != 7 for c in heading_cells)