fix: Bild-Endpoints fallen auf original zurueck fuer Sub-Sessions

Alle Bild-Endpoints (cropped, columns-overlay, rows-overlay,
words-overlay) suchten nur nach cropped/dewarped. Sub-Sessions haben
nur ein original-Bild. Neue Hilfsfunktion _get_base_image_png() mit
Fallback-Kette: cropped > dewarped > original.

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

klausur-service/backend/ocr_pipeline_api.py

@@ -94,6 +94,15 @@ router = APIRouter(prefix="/api/v1/ocr-pipeline", tags=["ocr-pipeline"])
 _cache: Dict[str, Dict[str, Any]] = {}
 
 
+async def _get_base_image_png(session_id: str) -> Optional[bytes]:
+    """Get the best available base image for a session (cropped > dewarped > original)."""
+    for img_type in ("cropped", "dewarped", "original"):
+        png_data = await get_session_image(session_id, img_type)
+        if png_data:
+            return png_data
+    return None
+
+
 async def _load_session_to_cache(session_id: str) -> Dict[str, Any]:
     """Load session from DB into cache, decoding PNGs to BGR arrays."""
     session = await get_session_db(session_id)
@@ -604,8 +613,11 @@ async def get_image(session_id: str, image_type: str):
         if image_type == "binarized" and cached.get("binarized_png"):
             return Response(content=cached["binarized_png"], media_type="image/png")
 
-    # Load from DB
-    data = await get_session_image(session_id, image_type)
+    # Load from DB — for cropped/dewarped, fall back through the chain
+    if image_type in ("cropped", "dewarped"):
+        data = await _get_base_image_png(session_id)
+    else:
+        data = await get_session_image(session_id, image_type)
     if not data:
         raise HTTPException(status_code=404, detail=f"Image '{image_type}' not available yet")
 
@@ -1398,12 +1410,10 @@ async def _get_columns_overlay(session_id: str) -> Response:
     if not column_result or not column_result.get("columns"):
         raise HTTPException(status_code=404, detail="No column data available")
 
-    # Load cropped image (preferred) or dewarped as fallback
-    base_png = await get_session_image(session_id, "cropped")
+    # Load best available base image (cropped > dewarped > original)
+    base_png = await _get_base_image_png(session_id)
     if not base_png:
-        base_png = await get_session_image(session_id, "dewarped")
-    if not base_png:
-        raise HTTPException(status_code=404, detail="No base image available (cropped/dewarped)")
+        raise HTTPException(status_code=404, detail="No base image available")
 
     arr = np.frombuffer(base_png, dtype=np.uint8)
     img = cv2.imdecode(arr, cv2.IMREAD_COLOR)
@@ -3160,12 +3170,10 @@ async def _get_rows_overlay(session_id: str) -> Response:
     if not row_result or not row_result.get("rows"):
         raise HTTPException(status_code=404, detail="No row data available")
 
-    # Load cropped image (preferred) or dewarped as fallback
-    base_png = await get_session_image(session_id, "cropped")
+    # Load best available base image (cropped > dewarped > original)
+    base_png = await _get_base_image_png(session_id)
     if not base_png:
-        base_png = await get_session_image(session_id, "dewarped")
-    if not base_png:
-        raise HTTPException(status_code=404, detail="No base image available (cropped/dewarped)")
+        raise HTTPException(status_code=404, detail="No base image available")
 
     arr = np.frombuffer(base_png, dtype=np.uint8)
     img = cv2.imdecode(arr, cv2.IMREAD_COLOR)
@@ -3247,12 +3255,10 @@ async def _get_words_overlay(session_id: str) -> Response:
     if not cells and not word_result.get("entries"):
         raise HTTPException(status_code=404, detail="No word data available")
 
-    # Load cropped image (preferred) or dewarped as fallback
-    base_png = await get_session_image(session_id, "cropped")
+    # Load best available base image (cropped > dewarped > original)
+    base_png = await _get_base_image_png(session_id)
     if not base_png:
-        base_png = await get_session_image(session_id, "dewarped")
-    if not base_png:
-        raise HTTPException(status_code=404, detail="No base image available (cropped/dewarped)")
+        raise HTTPException(status_code=404, detail="No base image available")
 
     arr = np.frombuffer(base_png, dtype=np.uint8)
     img = cv2.imdecode(arr, cv2.IMREAD_COLOR)