fix(dewarp): change manual slider to percentage (0-200%) instead of raw multiplier

The old -3.0 to +3.0 scale multiplied the full displacement map (up to ~79px)
directly, causing extreme distortion at values >1. New slider:
- 0% = no correction
- 100% = auto-detected correction (default)
- 200% = double correction
- Step size: 5%

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

klausur-service/backend/cv_vocab_pipeline.py

@@ -668,13 +668,13 @@ def dewarp_image_manual(img: np.ndarray, displacement_map: np.ndarray,
     Args:
         img: BGR image (deskewed, before dewarp).
         displacement_map: The displacement map from auto-dewarp.
-        scale: Manual scale factor (-3.0 to +3.0).
+        scale: Fraction of auto-detected correction (0.0 = none, 1.0 = auto, 2.0 = double).
 
     Returns:
         Corrected image.
     """
-    scale = max(-3.0, min(3.0, scale))
-    if abs(scale) < 0.01:
+    scale = max(0.0, min(2.0, scale))
+    if scale < 0.01:
         return img
     return _apply_displacement_map(img, displacement_map, scale=scale)
 

klausur-service/backend/ocr_pipeline_api.py

@@ -400,9 +400,9 @@ async def manual_dewarp(session_id: str, req: ManualDewarpRequest):
     if deskewed_bgr is None:
         raise HTTPException(status_code=400, detail="Deskew must be completed before dewarp")
 
-    scale = max(-3.0, min(3.0, req.scale))
+    scale = max(0.0, min(2.0, req.scale))
 
-    if displacement_map is None or abs(scale) < 0.01:
+    if displacement_map is None or scale < 0.01:
         # No displacement map or zero scale — use deskewed as-is
         dewarped_bgr = deskewed_bgr
     else: