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/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: