fix: merge nearby spine gaps + handle multi-page crop in frontend

Backend: merge gaps within 5% of image width — the spine area may have
thin ink strips splitting one physical gap into multiple detected gaps.
Only use gaps >= 2% width as split points.

Frontend: StepCrop now handles multi_page crop responses without
crashing on missing original_size/cropped_size fields.

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

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

@@ -108,10 +108,21 @@ export function StepCrop({ sessionId, onNext }: StepCropProps) {
       {cropResult && (
         <div className="bg-white dark:bg-gray-800 rounded-lg border border-gray-200 dark:border-gray-700 p-4">
           <div className="flex flex-wrap items-center gap-3 text-sm">
-            {cropResult.crop_applied ? (
+            {(cropResult as Record<string, unknown>).multi_page ? (
+              <>
+                <span className="inline-flex items-center gap-1.5 px-3 py-1 rounded-full bg-blue-50 dark:bg-blue-900/20 text-blue-700 dark:text-blue-400 text-xs font-medium">
+                  Mehrseitig: {(cropResult as Record<string, unknown>).page_count as number} Seiten erkannt
+                </span>
+                {((cropResult as Record<string, unknown>).sub_sessions as Array<{id: string; name: string; page_index: number}> | undefined)?.map((sub) => (
+                  <span key={sub.id} className="text-gray-400 text-xs">
+                    Seite {sub.page_index + 1}
+                  </span>
+                ))}
+              </>
+            ) : cropResult.crop_applied ? (
               <>
                 <span className="inline-flex items-center gap-1.5 px-3 py-1 rounded-full bg-amber-50 dark:bg-amber-900/20 text-amber-700 dark:text-amber-400 text-xs font-medium">
-                  ✂️ Zugeschnitten
+                  Zugeschnitten
                 </span>
                 {cropResult.detected_format && (
                   <>
@@ -126,10 +137,14 @@ export function StepCrop({ sessionId, onNext }: StepCropProps) {
                     </span>
                   </>
                 )}
-                <div className="h-4 w-px bg-gray-300 dark:bg-gray-600" />
-                <span className="text-gray-400 text-xs">
-                  {cropResult.original_size.width}x{cropResult.original_size.height} → {cropResult.cropped_size.width}x{cropResult.cropped_size.height}
-                </span>
+                {cropResult.original_size && cropResult.cropped_size && (
+                  <>
+                    <div className="h-4 w-px bg-gray-300 dark:bg-gray-600" />
+                    <span className="text-gray-400 text-xs">
+                      {cropResult.original_size.width}x{cropResult.original_size.height} → {cropResult.cropped_size.width}x{cropResult.cropped_size.height}
+                    </span>
+                  </>
+                )}
                 {cropResult.border_fractions && (
                   <>
                     <div className="h-4 w-px bg-gray-300 dark:bg-gray-600" />
@@ -141,7 +156,7 @@ export function StepCrop({ sessionId, onNext }: StepCropProps) {
               </>
             ) : (
               <span className="inline-flex items-center gap-1.5 px-3 py-1 rounded-full bg-green-50 dark:bg-green-900/20 text-green-700 dark:text-green-400 text-xs font-medium">
-                ✓ Kein Zuschnitt noetig
+                Kein Zuschnitt noetig
               </span>
             )}
             {cropResult.duration_seconds != null && (

klausur-service/backend/page_crop.py

@@ -100,12 +100,33 @@ def detect_page_splits(
     if not gaps:
         return []
 
+    # Merge nearby gaps (< 5% of width apart) — the spine area may have
+    # thin ink strips between multiple gap segments
+    merge_dist = max(20, int(w * 0.05))
+    merged: list = [gaps[0]]
+    for g in gaps[1:]:
+        prev = merged[-1]
+        prev_end = prev["x"] + prev["width"]
+        if g["x"] - prev_end < merge_dist:
+            # Merge: extend previous gap to cover both
+            new_end = g["x"] + g["width"]
+            prev["width"] = new_end - prev["x"]
+            prev["center"] = prev["x"] + prev["width"] // 2
+        else:
+            merged.append(g)
+    gaps = merged
+
     # Sort gaps by width (largest = most likely spine)
     gaps.sort(key=lambda g: g["width"], reverse=True)
 
-    # Use the widest gap(s) as split points
-    # For now: support up to N-1 gaps → N pages
-    split_points = sorted(g["center"] for g in gaps[:3])  # max 4 pages
+    # Use only gaps that are significant (>= 2% of image width)
+    significant_gaps = [g for g in gaps if g["width"] >= w * 0.02]
+    if not significant_gaps:
+        # Fall back to widest gap
+        significant_gaps = [gaps[0]]
+
+    # Use the significant gap(s) as split points
+    split_points = sorted(g["center"] for g in significant_gaps[:3])
 
     # Build page rectangles
     pages: list = []