Add Box-Grid-Review step (Step 11) to OCR pipeline

New pipeline step between Gutter Repair and Ground Truth that processes
embedded boxes (grammar tips, exercises) independently from the main grid.

Backend:
- cv_box_layout.py: classify_box_layout() detects flowing/columnar/
  bullet_list/header_only layout types per box
- build_box_zone_grid(): layout-aware grid building (single-column for
  flowing text, independent columns for tabular content)
- POST /sessions/{id}/build-box-grids endpoint with SmartSpellChecker
- Layout type overridable per box via request body

Frontend:
- StepBoxGridReview.tsx: shows each box with cropped image + editable
  GridTable. Layout type dropdown per box. Auto-builds on first load.
- Auto-skip when no boxes detected on page
- Pipeline steps updated: 13 steps (0-12), Ground Truth moved to 12

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

klausur-service/backend/grid_editor_api.py

@@ -2181,3 +2181,117 @@ async def gutter_repair_apply(session_id: str, request: Request):
     )
 
     return result
+
+
+# ---------------------------------------------------------------------------
+# Box-Grid-Review endpoints
+# ---------------------------------------------------------------------------
+
+@router.post("/sessions/{session_id}/build-box-grids")
+async def build_box_grids(session_id: str, request: Request):
+    """Rebuild grid structure for all box zones with layout-aware detection.
+
+    For each zone with zone_type='box':
+    1. Auto-detect layout type (flowing / columnar / bullet_list / header_only)
+    2. Build grid with layout-appropriate parameters
+    3. Apply SmartSpellChecker corrections
+    4. Store results back in grid_editor_result.zones[]
+
+    Optional body: { "overrides": { "2": "bullet_list" } }
+    Maps zone_index → forced layout_type.
+    """
+    session = await get_session_db(session_id)
+    if not session:
+        raise HTTPException(status_code=404, detail=f"Session {session_id} not found")
+
+    grid_data = session.get("grid_editor_result")
+    if not grid_data:
+        raise HTTPException(status_code=400, detail="No grid data. Run build-grid first.")
+
+    word_result = session.get("word_result") or {}
+    all_words = word_result.get("cells") or word_result.get("words") or []
+
+    body = {}
+    try:
+        body = await request.json()
+    except Exception:
+        pass
+    layout_overrides = body.get("overrides", {})
+
+    from cv_box_layout import classify_box_layout, build_box_zone_grid, _group_into_lines
+    from grid_editor_helpers import _words_in_zone
+
+    img_w = grid_data.get("image_width", 0)
+    img_h = grid_data.get("image_height", 0)
+
+    zones = grid_data.get("zones", [])
+    box_count = 0
+    spell_fixes = 0
+
+    for z in zones:
+        if z.get("zone_type") != "box":
+            continue
+
+        bbox = z.get("bbox_px", {})
+        bx, by = bbox.get("x", 0), bbox.get("y", 0)
+        bw, bh = bbox.get("w", 0), bbox.get("h", 0)
+
+        if bw <= 0 or bh <= 0:
+            continue
+
+        zone_idx = z.get("zone_index", 0)
+
+        # Filter words inside this box
+        zone_words = _words_in_zone(all_words, by, bh, bx, bw)
+        if not zone_words:
+            logger.info("Box zone %d: no words found in bbox", zone_idx)
+            continue
+
+        # Get layout override or auto-detect
+        forced_layout = layout_overrides.get(str(zone_idx))
+
+        # Build box grid
+        box_grid = build_box_zone_grid(
+            zone_words, bx, by, bw, bh,
+            zone_idx, img_w, img_h,
+            layout_type=forced_layout,
+        )
+
+        # Apply SmartSpellChecker to all box cells
+        try:
+            from smart_spell import SmartSpellChecker
+            ssc = SmartSpellChecker()
+            for cell in box_grid.get("cells", []):
+                text = cell.get("text", "")
+                if not text:
+                    continue
+                result = ssc.correct_text(text, lang="auto")
+                if result.changed:
+                    cell["text"] = result.corrected
+                    spell_fixes += 1
+        except ImportError:
+            pass
+
+        # Update zone data with new grid
+        z["columns"] = box_grid["columns"]
+        z["rows"] = box_grid["rows"]
+        z["cells"] = box_grid["cells"]
+        z["header_rows"] = box_grid.get("header_rows", [])
+        z["box_layout_type"] = box_grid.get("box_layout_type", "flowing")
+        z["box_grid_reviewed"] = False
+        box_count += 1
+
+    # Save updated grid back
+    await update_session_db(session_id, grid_editor_result=grid_data)
+
+    logger.info(
+        "build-box-grids session %s: %d box zones rebuilt, %d spell fixes",
+        session_id, box_count, spell_fixes,
+    )
+
+    return {
+        "session_id": session_id,
+        "box_zones_rebuilt": box_count,
+        "spell_fixes": spell_fixes,
+        "zones": zones,
+    }