fix: Sub-Sessions original_bgr als cropped_bgr promoten

Spalten-/Zeilen-/Woerter-Erkennung suchen nach cropped_bgr oder
dewarped_bgr. Bei Sub-Sessions existiert nur original_bgr (der
Box-Ausschnitt). Jetzt wird original_bgr automatisch als cropped_bgr
gesetzt, sowohl im Cache-Aufbau als auch bei der Erstellung.

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

klausur-service/backend/ocr_pipeline_api.py

@@ -127,6 +127,12 @@ async def _load_session_to_cache(session_id: str) -> Dict[str, Any]:
             bgr = cv2.imdecode(arr, cv2.IMREAD_COLOR)
             cache_entry[bgr_key] = bgr
 
+    # Sub-sessions: original image IS the cropped box region.
+    # Promote original_bgr to cropped_bgr so downstream steps find it.
+    if session.get("parent_session_id") and cache_entry["original_bgr"] is not None:
+        if cache_entry["cropped_bgr"] is None and cache_entry["dewarped_bgr"] is None:
+            cache_entry["cropped_bgr"] = cache_entry["original_bgr"]
+
     _cache[session_id] = cache_entry
     return cache_entry
 
@@ -465,13 +471,16 @@ async def create_box_sessions(session_id: str):
         )
 
         # Cache the BGR for immediate processing
+        # Promote original to cropped so column/row/word detection finds it
+        box_bgr = crop.copy()
         _cache[sub_id] = {
             "id": sub_id,
             "filename": session.get("filename", "box-crop.png"),
             "name": sub_name,
-            "original_bgr": crop.copy(),
+            "parent_session_id": session_id,
+            "original_bgr": box_bgr,
             "oriented_bgr": None,
-            "cropped_bgr": None,
+            "cropped_bgr": box_bgr,
             "deskewed_bgr": None,
             "dewarped_bgr": None,
             "orientation_result": None,