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/ImageCompareView.tsx

@@ -9,6 +9,7 @@ interface ImageCompareViewProps {
   originalUrl: string | null
   deskewedUrl: string | null
   showGrid: boolean
+  showGridLeft?: boolean
   showBinarized: boolean
   binarizedUrl: string | null
   leftLabel?: string
@@ -77,6 +78,7 @@ export function ImageCompareView({
   originalUrl,
   deskewedUrl,
   showGrid,
+  showGridLeft,
   showBinarized,
   binarizedUrl,
   leftLabel,
@@ -95,12 +97,15 @@ export function ImageCompareView({
         <div className="relative bg-gray-100 dark:bg-gray-900 rounded-lg overflow-hidden border border-gray-200 dark:border-gray-700"
              style={{ aspectRatio: '210/297' }}>
           {originalUrl && !leftError ? (
-            <img
-              src={originalUrl}
-              alt="Original Scan"
-              className="w-full h-full object-contain"
-              onError={() => setLeftError(true)}
-            />
+            <>
+              <img
+                src={originalUrl}
+                alt="Original Scan"
+                className="w-full h-full object-contain"
+                onError={() => setLeftError(true)}
+              />
+              {showGridLeft && <MmGridOverlay />}
+            </>
           ) : (
             <div className="flex items-center justify-center h-full text-gray-400">
               {leftError ? 'Fehler beim Laden' : 'Noch kein Bild'}

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

@@ -1,6 +1,6 @@
 'use client'
 
-import { useCallback, useState } from 'react'
+import { useCallback, useEffect, useState } from 'react'
 import type { DeskewGroundTruth, DeskewResult, SessionInfo } from '@/app/(admin)/ai/ocr-pipeline/types'
 import { DeskewControls } from './DeskewControls'
 import { ImageCompareView } from './ImageCompareView'
@@ -8,10 +8,11 @@ import { ImageCompareView } from './ImageCompareView'
 const KLAUSUR_API = '/klausur-api'
 
 interface StepDeskewProps {
+  sessionId?: string | null
   onNext: (sessionId: string) => void
 }
 
-export function StepDeskew({ onNext }: StepDeskewProps) {
+export function StepDeskew({ sessionId: existingSessionId, onNext }: StepDeskewProps) {
   const [session, setSession] = useState<SessionInfo | null>(null)
   const [deskewResult, setDeskewResult] = useState<DeskewResult | null>(null)
   const [uploading, setUploading] = useState(false)
@@ -22,6 +23,42 @@ export function StepDeskew({ onNext }: StepDeskewProps) {
   const [error, setError] = useState<string | null>(null)
   const [dragOver, setDragOver] = useState(false)
 
+  // Reload session data when navigating back from a later step
+  useEffect(() => {
+    if (!existingSessionId || session) return
+
+    const loadSession = async () => {
+      try {
+        const res = await fetch(`${KLAUSUR_API}/api/v1/ocr-pipeline/sessions/${existingSessionId}`)
+        if (!res.ok) return
+        const data = await res.json()
+
+        const sessionInfo: SessionInfo = {
+          session_id: data.session_id,
+          filename: data.filename,
+          image_width: data.image_width,
+          image_height: data.image_height,
+          original_image_url: `${KLAUSUR_API}${data.original_image_url}`,
+        }
+        setSession(sessionInfo)
+
+        // Reconstruct deskew result from session data
+        if (data.deskew_result) {
+          const dr: DeskewResult = {
+            ...data.deskew_result,
+            deskewed_image_url: `${KLAUSUR_API}/api/v1/ocr-pipeline/sessions/${existingSessionId}/image/deskewed`,
+            binarized_image_url: `${KLAUSUR_API}/api/v1/ocr-pipeline/sessions/${existingSessionId}/image/binarized`,
+          }
+          setDeskewResult(dr)
+        }
+      } catch (e) {
+        console.error('Failed to reload session:', e)
+      }
+    }
+
+    loadSession()
+  }, [existingSessionId, session])
+
   const handleUpload = useCallback(async (file: File) => {
     setUploading(true)
     setError(null)

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)' : ''}`}
       />