refactor: Crop nach Deskew/Dewarp verschieben + content-basierter Buchscan-Crop

Pipeline-Reihenfolge neu: Orientierung → Begradigung → Entzerrung → Zuschneiden → Spalten...
Crop arbeitet jetzt auf dem bereits geraden Bild, was bessere Ergebnisse liefert.

page_crop.py komplett ersetzt: Adaptive Threshold + 4-Kanten-Erkennung
(Buchruecken-Schatten links, Ink-Projektion fuer alle Raender) statt
Otsu + groesste Kontur.

Backend: Step-Nummern, Input-Bilder, Reprocess-Kaskade angepasst.
Frontend: PIPELINE_STEPS umgeordnet, Switch-Cases, Vorher-Bilder aktualisiert.

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

klausur-service/backend/orientation_crop_api.py

@@ -1,8 +1,8 @@
 """
-Orientation & Crop API - Steps 1-2 of the OCR Pipeline.
+Orientation & Crop API - Steps 1 and 4 of the OCR Pipeline.
 
 Step 1: Orientation detection (fix 90/180/270 degree rotations)
-Step 2: Page cropping (remove scanner borders, detect paper format)
+Step 4 (UI index 3): Page cropping (after deskew + dewarp, so the image is straight)
 
 These endpoints were extracted from the main pipeline to keep files manageable.
 """
@@ -161,21 +161,24 @@ async def detect_orientation(session_id: str):
 
 
 # ---------------------------------------------------------------------------
-# Step 2: Crop
+# Step 4 (UI index 3): Crop — runs after deskew + dewarp
 # ---------------------------------------------------------------------------
 
 @router.post("/sessions/{session_id}/crop")
 async def auto_crop(session_id: str):
-    """Auto-detect and crop scanner borders.
+    """Auto-detect and crop scanner/book borders.
 
-    Reads the oriented image (or original if no orientation step),
-    detects the page boundary and crops.
+    Reads the dewarped image (post-deskew + dewarp, so the page is straight).
+    Falls back to oriented → original if earlier steps were skipped.
     """
     cached = await _ensure_cached(session_id)
 
-    # Use oriented image if available, else original
-    oriented = cached.get("oriented_bgr")
-    img_bgr = oriented if oriented is not None else cached.get("original_bgr")
+    # Use dewarped (preferred), fall back to oriented, then original
+    img_bgr = next(
+        (v for k in ("dewarped_bgr", "oriented_bgr", "original_bgr")
+         if (v := cached.get(k)) is not None),
+        None,
+    )
     if img_bgr is None:
         raise HTTPException(status_code=400, detail="No image available for cropping")
 
@@ -199,7 +202,7 @@ async def auto_crop(session_id: str):
         session_id,
         cropped_png=cropped_png,
         crop_result=crop_info,
-        current_step=3,
+        current_step=5,
     )
 
     logger.info(
@@ -237,8 +240,11 @@ async def manual_crop(session_id: str, req: ManualCropRequest):
     """Manually crop using percentage coordinates."""
     cached = await _ensure_cached(session_id)
 
-    oriented = cached.get("oriented_bgr")
-    img_bgr = oriented if oriented is not None else cached.get("original_bgr")
+    img_bgr = next(
+        (v for k in ("dewarped_bgr", "oriented_bgr", "original_bgr")
+         if (v := cached.get(k)) is not None),
+        None,
+    )
     if img_bgr is None:
         raise HTTPException(status_code=400, detail="No image available for cropping")
 
@@ -278,7 +284,7 @@ async def manual_crop(session_id: str, req: ManualCropRequest):
         session_id,
         cropped_png=cropped_png,
         crop_result=crop_result,
-        current_step=3,
+        current_step=5,
     )
 
     ch, cw = cropped_bgr.shape[:2]
@@ -293,17 +299,20 @@ async def manual_crop(session_id: str, req: ManualCropRequest):
 
 @router.post("/sessions/{session_id}/crop/skip")
 async def skip_crop(session_id: str):
-    """Skip cropping — use oriented (or original) image as-is."""
+    """Skip cropping — use dewarped (or oriented/original) image as-is."""
     cached = await _ensure_cached(session_id)
 
-    oriented = cached.get("oriented_bgr")
-    img_bgr = oriented if oriented is not None else cached.get("original_bgr")
+    img_bgr = next(
+        (v for k in ("dewarped_bgr", "oriented_bgr", "original_bgr")
+         if (v := cached.get(k)) is not None),
+        None,
+    )
     if img_bgr is None:
         raise HTTPException(status_code=400, detail="No image available")
 
     h, w = img_bgr.shape[:2]
 
-    # Store the oriented image as cropped (identity crop)
+    # Store the dewarped image as cropped (identity crop)
     success, png_buf = cv2.imencode(".png", img_bgr)
     cropped_png = png_buf.tobytes() if success else b""
 
@@ -321,7 +330,7 @@ async def skip_crop(session_id: str):
         session_id,
         cropped_png=cropped_png,
         crop_result=crop_result,
-        current_step=3,
+        current_step=5,
     )
 
     return {