Filter false-positive boxes in header/footer margins

Boxes whose vertical center falls within top/bottom 7% of image
height are filtered out (page numbers, unit headers, running footers).
At typical scan resolutions, 7% ≈ 2.5cm margin.

Fixes: "Box 1" containing just "3" from "Unit 3" page header being
incorrectly treated as an embedded box.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

klausur-service/backend/grid_editor_api.py

@@ -2221,6 +2221,28 @@ async def build_box_grids(session_id: str, request: Request):
     if not detected_boxes:
         return {"session_id": session_id, "box_zones_rebuilt": 0, "spell_fixes": 0, "message": "No boxes detected"}
 
+    # Filter out false-positive boxes in header/footer margins.
+    # Textbook pages have ~2.5cm margins at top/bottom. At typical scan
+    # resolutions (150-300 DPI), that's roughly 5-10% of image height.
+    # A box whose vertical CENTER falls within the top or bottom 7% of
+    # the image is likely a page number, unit header, or running footer.
+    img_h_for_filter = grid_data.get("image_height", 0) or word_result.get("image_height", 0)
+    if img_h_for_filter > 0:
+        margin_frac = 0.07  # 7% of image height
+        margin_top = img_h_for_filter * margin_frac
+        margin_bottom = img_h_for_filter * (1 - margin_frac)
+        filtered = []
+        for box in detected_boxes:
+            by = box.get("y", 0)
+            bh = box.get("h", 0)
+            box_center_y = by + bh / 2
+            if box_center_y < margin_top or box_center_y > margin_bottom:
+                logger.info("build-box-grids: skipping header/footer box at y=%d h=%d (center=%.0f, margins=%.0f/%.0f)",
+                            by, bh, box_center_y, margin_top, margin_bottom)
+                continue
+            filtered.append(box)
+        detected_boxes = filtered
+
     body = {}
     try:
         body = await request.json()