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

@@ -92,15 +92,24 @@ export default function OcrPipelinePage() {
       let uiStep = Math.max(0, dbStep - 1)
       const skipSteps = [...(savedDocType?.skip_steps || [])]
 
-      // Sub-sessions: image is already cropped, skip pre-processing steps
-      // Jump directly to columns (UI step 4) unless already further ahead
+      // Sub-session handling depends on how they were created:
+      // - Crop-based (current_step >= 5): image already cropped, skip all pre-processing
+      // - Page-split (current_step 2): orientation done on parent, skip only orientation
+      // - Page-split from original (current_step 1): needs full pipeline
       const isSubSession = !!data.parent_session_id
-      const SUB_SESSION_SKIP = ['orientation', 'deskew', 'dewarp', 'crop']
       if (isSubSession) {
-        for (const s of SUB_SESSION_SKIP) {
-          if (!skipSteps.includes(s)) skipSteps.push(s)
+        if (dbStep >= 5) {
+          // Crop-based sub-sessions: image already cropped
+          const SUB_SESSION_SKIP = ['orientation', 'deskew', 'dewarp', 'crop']
+          for (const s of SUB_SESSION_SKIP) {
+            if (!skipSteps.includes(s)) skipSteps.push(s)
+          }
+          if (uiStep < 4) uiStep = 4 // columns step (index 4)
+        } else if (dbStep >= 2) {
+          // Page-split sub-session: parent orientation applied, skip only orientation
+          if (!skipSteps.includes('orientation')) skipSteps.push('orientation')
         }
-        if (uiStep < 4) uiStep = 4 // columns step (index 4)
+        // dbStep === 1: page-split from original image, needs full pipeline
       }
 
       setSteps(
@@ -245,6 +254,13 @@ export default function OcrPipelinePage() {
     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
+    }
+
     handleNext()
   }
 
@@ -365,7 +381,7 @@ export default function OcrPipelinePage() {
   const renderStep = () => {
     switch (currentStep) {
       case 0:
-        return <StepOrientation sessionId={sessionId} onNext={handleOrientationComplete} />
+        return <StepOrientation sessionId={sessionId} onNext={handleOrientationComplete} onSubSessionsCreated={handleBoxSessionsCreated} />
       case 1:
         return <StepDeskew sessionId={sessionId} onNext={handleNext} />
       case 2: