Add double-page spread detection to frontend pipeline

After orientation detection, the frontend now automatically calls the
page-split endpoint. When a double-page book spread is detected, two
sub-sessions are created and each goes through the full pipeline
(deskew/dewarp/crop) independently — essential because each page of a
spread tilts differently due to the spine.

Frontend changes:
- StepOrientation: calls POST /page-split after orientation, shows
  split info ("Doppelseite erkannt"), notifies parent of sub-sessions
- page.tsx: distinguishes page-split sub-sessions (current_step < 5)
  from crop-based sub-sessions (current_step >= 5). Page-split subs
  only skip orientation, not deskew/dewarp/crop.
- page.tsx: handleOrientationComplete opens first sub-session when
  page-split was detected

Backend changes (orientation_crop_api.py):
- page-split endpoint falls back to original image when orientation
  rotated a landscape spread to portrait
- start_step parameter: 1 if split from original, 2 if from oriented

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

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

@@ -1,19 +1,29 @@
 'use client'
 
 import { useCallback, useEffect, useState } from 'react'
-import type { OrientationResult, SessionInfo } from '@/app/(admin)/ai/ocr-pipeline/types'
+import type { OrientationResult, SessionInfo, SubSession } from '@/app/(admin)/ai/ocr-pipeline/types'
 import { ImageCompareView } from './ImageCompareView'
 
 const KLAUSUR_API = '/klausur-api'
 
+interface PageSplitResult {
+  multi_page: boolean
+  page_count?: number
+  sub_sessions?: { id: string; name: string; page_index: number }[]
+  used_original?: boolean
+  duration_seconds?: number
+}
+
 interface StepOrientationProps {
   sessionId?: string | null
   onNext: (sessionId: string) => void
+  onSubSessionsCreated?: (subs: SubSession[]) => void
 }
 
-export function StepOrientation({ sessionId: existingSessionId, onNext }: StepOrientationProps) {
+export function StepOrientation({ sessionId: existingSessionId, onNext, onSubSessionsCreated }: StepOrientationProps) {
   const [session, setSession] = useState<SessionInfo | null>(null)
   const [orientationResult, setOrientationResult] = useState<OrientationResult | null>(null)
+  const [pageSplitResult, setPageSplitResult] = useState<PageSplitResult | null>(null)
   const [uploading, setUploading] = useState(false)
   const [detecting, setDetecting] = useState(false)
   const [error, setError] = useState<string | null>(null)
@@ -92,13 +102,38 @@ export function StepOrientation({ sessionId: existingSessionId, onNext }: StepOr
         corrected: orientData.corrected,
         duration_seconds: orientData.duration_seconds,
       })
+
+      // Auto-trigger page-split detection (double-page book spreads)
+      try {
+        const splitRes = await fetch(
+          `${KLAUSUR_API}/api/v1/ocr-pipeline/sessions/${data.session_id}/page-split`,
+          { method: 'POST' },
+        )
+        if (splitRes.ok) {
+          const splitData: PageSplitResult = await splitRes.json()
+          setPageSplitResult(splitData)
+          if (splitData.multi_page && splitData.sub_sessions && onSubSessionsCreated) {
+            onSubSessionsCreated(
+              splitData.sub_sessions.map((s) => ({
+                id: s.id,
+                name: s.name,
+                box_index: s.page_index,
+                current_step: splitData.used_original ? 1 : 2,
+              }))
+            )
+          }
+        }
+      } catch (e) {
+        console.error('Page-split detection failed:', e)
+        // Not critical — continue as single page
+      }
     } catch (e) {
       setError(e instanceof Error ? e.message : 'Unbekannter Fehler')
     } finally {
       setUploading(false)
       setDetecting(false)
     }
-  }, [sessionName])
+  }, [sessionName, onSubSessionsCreated])
 
   const handleDrop = useCallback((e: React.DragEvent) => {
     e.preventDefault()
@@ -225,6 +260,29 @@ export function StepOrientation({ sessionId: existingSessionId, onNext }: StepOr
         </div>
       )}
 
+      {/* Page-split result */}
+      {pageSplitResult?.multi_page && (
+        <div className="bg-blue-50 dark:bg-blue-900/20 rounded-lg border border-blue-200 dark:border-blue-700 p-4">
+          <div className="text-sm font-medium text-blue-700 dark:text-blue-300">
+            Doppelseite erkannt — {pageSplitResult.page_count} Seiten
+          </div>
+          <p className="text-xs text-blue-600 dark:text-blue-400 mt-1">
+            Jede Seite wird einzeln durch die Pipeline (Begradigung, Entzerrung, Zuschnitt, ...) verarbeitet.
+            {pageSplitResult.used_original && ' (Seitentrennung auf dem Originalbild, da die Orientierung die Doppelseite gedreht hat.)'}
+          </p>
+          <div className="flex gap-2 mt-2">
+            {pageSplitResult.sub_sessions?.map((s) => (
+              <span
+                key={s.id}
+                className="text-xs px-2 py-1 rounded-md bg-blue-100 dark:bg-blue-800/40 text-blue-700 dark:text-blue-300"
+              >
+                {s.name}
+              </span>
+            ))}
+          </div>
+        </div>
+      )}
+
       {/* Next button */}
       {orientationResult && (
         <div className="flex justify-end">
@@ -232,7 +290,7 @@ export function StepOrientation({ sessionId: existingSessionId, onNext }: StepOr
             onClick={() => onNext(session.session_id)}
             className="px-6 py-2 bg-teal-600 text-white rounded-lg hover:bg-teal-700 font-medium transition-colors"
           >
-            Weiter &rarr;
+            {pageSplitResult?.multi_page ? 'Seiten verarbeiten' : 'Weiter'} &rarr;
           </button>
         </div>
       )}