feat(ocr-pipeline): ground-truth comparison tool for column detection

Side-by-side view: auto result (readonly) vs GT editor where teacher
draws correct columns. Diff table shows Auto vs GT with IoU matching.
GT data persisted per session for algorithm tuning.

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

klausur-service/backend/ocr_pipeline_api.py

@@ -135,6 +135,7 @@ class ManualColumnsRequest(BaseModel):
 
 class ColumnGroundTruthRequest(BaseModel):
     is_correct: bool
+    corrected_columns: Optional[List[Dict[str, Any]]] = None
     notes: Optional[str] = None
 
 
@@ -711,6 +712,7 @@ async def save_column_ground_truth(session_id: str, req: ColumnGroundTruthReques
     ground_truth = session.get("ground_truth") or {}
     gt = {
         "is_correct": req.is_correct,
+        "corrected_columns": req.corrected_columns,
         "notes": req.notes,
         "saved_at": datetime.utcnow().isoformat(),
         "column_result": session.get("column_result"),
@@ -725,6 +727,25 @@ async def save_column_ground_truth(session_id: str, req: ColumnGroundTruthReques
     return {"session_id": session_id, "ground_truth": gt}
 
 
+@router.get("/sessions/{session_id}/ground-truth/columns")
+async def get_column_ground_truth(session_id: str):
+    """Retrieve saved ground truth for column detection, including auto vs GT diff."""
+    session = await get_session_db(session_id)
+    if not session:
+        raise HTTPException(status_code=404, detail=f"Session {session_id} not found")
+
+    ground_truth = session.get("ground_truth") or {}
+    columns_gt = ground_truth.get("columns")
+    if not columns_gt:
+        raise HTTPException(status_code=404, detail="No column ground truth saved")
+
+    return {
+        "session_id": session_id,
+        "columns_gt": columns_gt,
+        "columns_auto": session.get("column_result"),
+    }
+
+
 async def _get_columns_overlay(session_id: str) -> Response:
     """Generate dewarped image with column borders drawn on it."""
     session = await get_session_db(session_id)