Fix page-split: fetch sub-sessions from API instead of React state

handleOrientationComplete was checking subSessions from React state,
but due to batching the state was still empty when the user clicked
"Seiten verarbeiten". Now fetches session data directly from the API
to reliably detect sub-sessions and auto-open the first one.

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

admin-lehrer/app/(admin)/ai/ocr-pipeline/page.tsx

@@ -250,15 +250,31 @@ export default function OcrPipelinePage() {
     setCurrentStep(nextStep)
   }
 
-  const handleOrientationComplete = (sid: string) => {
+  const handleOrientationComplete = async (sid: string) => {
     setSessionId(sid)
-    // Reload session list to show the new session
     loadSessions()
 
-    // If page-split created sub-sessions, open the first one
-    if (subSessions.length > 0) {
-      openSession(subSessions[0].id, true)
-      return
+    // Check for page-split sub-sessions directly from API
+    // (React state may not be committed yet due to batching)
+    try {
+      const res = await fetch(`${KLAUSUR_API}/api/v1/ocr-pipeline/sessions/${sid}`)
+      if (res.ok) {
+        const data = await res.json()
+        if (data.sub_sessions?.length > 0) {
+          const subs: SubSession[] = data.sub_sessions.map((s: SubSession) => ({
+            id: s.id,
+            name: s.name,
+            box_index: s.box_index,
+            current_step: s.current_step,
+          }))
+          setSubSessions(subs)
+          setParentSessionId(sid)
+          openSession(subs[0].id, true)
+          return
+        }
+      }
+    } catch (e) {
+      console.error('Failed to check for sub-sessions:', e)
     }
 
     handleNext()