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/app/(admin)/ai/ocr-pipeline/types.ts

@@ -37,17 +37,16 @@ export interface DeskewGroundTruth {
 
 export interface DewarpResult {
   session_id: string
-  method_used: 'vertical_edge' | 'text_baseline' | 'manual' | 'none'
-  curvature_px: number
+  method_used: 'vertical_edge' | 'manual' | 'none'
+  shear_degrees: number
   confidence: number
   duration_seconds: number
   dewarped_image_url: string
-  scale_applied?: number
 }
 
 export interface DewarpGroundTruth {
   is_correct: boolean
-  corrected_scale?: number
+  corrected_shear?: number
   notes?: string
 }