feat(ocr-pipeline): 6 systematic improvements for robustness, performance & UX

1. Unit tests: 76 new parametrized tests for noise filter, phonetic detection,
   cell text cleaning, and row merging (116 total, all green)
2. Continuation-row merge: detect multi-line vocab entries where text wraps
   (lowercase EN + empty DE) and merge into previous entry
3. Empty DE fallback: secondary PSM=7 OCR pass for cells missed by PSM=6
4. Batch-OCR: collect empty cells per column, run single Tesseract call on
   column strip instead of per-cell (~66% fewer calls for 3+ empty cells)
5. StepReconstruction UI: font scaling via naturalHeight, empty EN/DE field
   highlighting, undo/redo (Ctrl+Z), per-cell reset button
6. Session reprocess: POST /sessions/{id}/reprocess endpoint to re-run from
   any step, with reprocess button on completed pipeline steps

Also fixes pre-existing dewarp_image tuple unpacking bug in run_cv_pipeline
and updates dewarp tests to match current (image, info) return signature.

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

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

@@ -160,6 +160,29 @@ export default function OcrPipelinePage() {
     8: 'Validierung',
   }
 
+  const reprocessFromStep = useCallback(async (uiStep: number) => {
+    if (!sessionId) return
+    const dbStep = uiStep + 1 // UI is 0-indexed, DB is 1-indexed
+    if (!confirm(`Ab Schritt ${dbStep} (${stepNames[dbStep] || '?'}) neu verarbeiten? Nachfolgende Daten werden geloescht.`)) return
+    try {
+      const res = await fetch(`${KLAUSUR_API}/api/v1/ocr-pipeline/sessions/${sessionId}/reprocess`, {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ from_step: dbStep }),
+      })
+      if (!res.ok) {
+        const data = await res.json().catch(() => ({}))
+        console.error('Reprocess failed:', data.detail || res.status)
+        return
+      }
+      // Reset UI steps
+      goToStep(uiStep)
+    } catch (e) {
+      console.error('Reprocess error:', e)
+    }
+  // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [sessionId, goToStep])
+
   const renderStep = () => {
     switch (currentStep) {
       case 0:
@@ -291,7 +314,7 @@ export default function OcrPipelinePage() {
         </div>
       )}
 
-      <PipelineStepper steps={steps} currentStep={currentStep} onStepClick={handleStepClick} />
+      <PipelineStepper steps={steps} currentStep={currentStep} onStepClick={handleStepClick} onReprocess={sessionId ? reprocessFromStep : undefined} />
 
       <div className="min-h-[400px]">{renderStep()}</div>
     </div>