fix(ocr-pipeline): dewarp visibility, grid on both sides, session persistence

- Fix dewarp method selection: prefer methods with >5px curvature over
  higher confidence (vertical_edge 79px was being ignored for text_baseline 2px)
- Add grid overlay on left image in Dewarp step for side-by-side comparison
- Add GET /sessions/{id} endpoint to reload session data
- StepDeskew accepts sessionId prop to restore state when navigating back
- SessionInfo type extended with optional deskew_result and dewarp_result

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

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

@@ -123,9 +123,10 @@ export function StepDewarp({ sessionId, onNext }: StepDewarpProps) {
         originalUrl={deskewedUrl}
         deskewedUrl={dewarpedUrl}
         showGrid={showGrid}
+        showGridLeft={showGrid}
         showBinarized={false}
         binarizedUrl={null}
-        leftLabel="Begradigt (nach Deskew)"
+        leftLabel={`Begradigt (nach Deskew)${showGrid ? ' + Raster' : ''}`}
         rightLabel={`Entzerrt${showGrid ? ' + Raster (mm)' : ''}`}
       />