Add "OCR neu + Grid" button to Grid Review

New endpoint POST /sessions/{id}/rerun-ocr-and-build-grid that:
1. Runs scan quality assessment
2. Applies CLAHE enhancement if degraded (controlled by enhance toggle)
3. Re-runs dual-engine OCR (RapidOCR + Tesseract) with min_conf filter
4. Merges OCR results and stores updated word_result
5. Builds grid with max_columns constraint

Frontend: Orange "OCR neu + Grid" button in GridToolbar.
Unlike "Neu berechnen" (which only rebuilds grid from existing words),
this button re-runs the full OCR pipeline with quality settings.

Now CLAHE toggle actually has an effect — it enhances the image
before OCR runs, not after.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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

@@ -67,6 +67,7 @@ export function StepGridReview({ sessionId, onNext, saveRef }: StepGridReviewPro
     setOcrMaxCols,
     ocrMinConf,
     setOcrMinConf,
+    rerunOcr,
   } = useGridEditor(sessionId)
 
   const [showImage, setShowImage] = useState(true)
@@ -336,6 +337,14 @@ export function StepGridReview({ sessionId, onNext, saveRef }: StepGridReviewPro
           onIpaModeChange={setIpaMode}
           onSyllableModeChange={setSyllableMode}
         />
+        <button
+          onClick={rerunOcr}
+          disabled={loading}
+          className="ml-2 px-3 py-1.5 text-xs font-medium rounded border border-orange-300 dark:border-orange-700 bg-orange-50 dark:bg-orange-900/20 text-orange-700 dark:text-orange-300 hover:bg-orange-100 dark:hover:bg-orange-900/40 transition-colors disabled:opacity-50"
+          title="OCR komplett neu ausfuehren mit aktuellen Quality-Step-Einstellungen (CLAHE, MinConf), dann Grid neu bauen"
+        >
+          {loading ? 'OCR laeuft...' : 'OCR neu + Grid'}
+        </button>
       </div>
 
       {/* Split View: Image left + Grid right */}