refactor(dewarp): replace displacement map with affine shear correction

The old displacement-map approach shifted entire rows by a parabolic
profile, creating a circle/barrel distortion. The actual problem is
a linear vertical shear: after deskew aligns horizontal lines, the
vertical column edges are still tilted by ~0.5°.

New approach:
- Detect shear angle from strongest vertical edge slope (not curvature)
- Apply cv2.warpAffine shear to straighten vertical features
- Manual slider: -2.0° to +2.0° in 0.05° steps
- Slider initializes to auto-detected shear angle
- Ground truth question: "Spalten vertikal ausgerichtet?"

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

admin-lehrer/components/ocr-pipeline/StepDewarp.tsx

@@ -47,7 +47,7 @@ export function StepDewarp({ sessionId, onNext }: StepDewarpProps) {
     runDewarp()
   }, [sessionId, dewarpResult])
 
-  const handleManualDewarp = useCallback(async (scale: number) => {
+  const handleManualDewarp = useCallback(async (shearDegrees: number) => {
     if (!sessionId) return
     setApplying(true)
     setError(null)
@@ -56,7 +56,7 @@ export function StepDewarp({ sessionId, onNext }: StepDewarpProps) {
       const res = await fetch(`${KLAUSUR_API}/api/v1/ocr-pipeline/sessions/${sessionId}/dewarp/manual`, {
         method: 'POST',
         headers: { 'Content-Type': 'application/json' },
-        body: JSON.stringify({ scale }),
+        body: JSON.stringify({ shear_degrees: shearDegrees }),
       })
       if (!res.ok) throw new Error('Manuelle Entzerrung fehlgeschlagen')
 
@@ -66,7 +66,7 @@ export function StepDewarp({ sessionId, onNext }: StepDewarpProps) {
           ? {
               ...prev,
               method_used: data.method_used,
-              scale_applied: data.scale_applied,
+              shear_degrees: data.shear_degrees,
               dewarped_image_url: `${KLAUSUR_API}${data.dewarped_image_url}?t=${Date.now()}`,
             }
           : null,