Add Vision-LLM OCR Fusion (Step 4) for degraded scans

New module vision_ocr_fusion.py: Sends scan image + OCR word
coordinates + document type to Qwen2.5-VL 32B. The LLM reads
the image visually while using OCR positions as structural hints.

Key features:
- Document-type-aware prompts (Vokabelseite, Woerterbuch, etc.)
- OCR words grouped into lines with x/y coordinates in prompt
- Low-confidence words marked with (?) for LLM attention
- Continuation row merging instructions in prompt
- JSON response parsing with markdown code block handling
- Fallback to original OCR on any error

Frontend (admin-lehrer Grid Review):
- "Vision-LLM" checkbox toggle
- "Typ" dropdown (Vokabelseite, Woerterbuch, etc.)
- Steps 1-3 defaults set to inactive

Activate: Check "Vision-LLM", select document type, click "OCR neu + Grid".

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

admin-lehrer/components/grid-editor/useGridEditor.ts

@@ -28,10 +28,14 @@ export function useGridEditor(sessionId: string | null) {
   const [ipaMode, setIpaMode] = useState<IpaMode>('auto')
   const [syllableMode, setSyllableMode] = useState<SyllableMode>('auto')
 
-  // OCR Quality Steps (A/B testing toggles)
-  const [ocrEnhance, setOcrEnhance] = useState(true)
-  const [ocrMaxCols, setOcrMaxCols] = useState(0) // 0 = unlimited (admin pipeline default)
-  const [ocrMinConf, setOcrMinConf] = useState(0) // 0 = auto from quality score
+  // OCR Quality Steps (A/B testing toggles — defaults off for now)
+  const [ocrEnhance, setOcrEnhance] = useState(false)
+  const [ocrMaxCols, setOcrMaxCols] = useState(0)
+  const [ocrMinConf, setOcrMinConf] = useState(0)
+
+  // Vision-LLM Fusion (Step 4)
+  const [visionFusion, setVisionFusion] = useState(false)
+  const [documentCategory, setDocumentCategory] = useState('vokabelseite')
 
   // Undo/redo stacks store serialized zone arrays
   const undoStack = useRef<string[]>([])
@@ -92,6 +96,8 @@ export function useGridEditor(sessionId: string | null) {
       params.set('enhance', String(ocrEnhance))
       if (ocrMaxCols > 0) params.set('max_cols', String(ocrMaxCols))
       if (ocrMinConf > 0) params.set('min_conf', String(ocrMinConf))
+      params.set('vision_fusion', String(visionFusion))
+      if (documentCategory) params.set('doc_category', documentCategory)
       const res = await fetch(
         `${KLAUSUR_API}/api/v1/ocr-pipeline/sessions/${sessionId}/rerun-ocr-and-build-grid?${params}`,
         { method: 'POST' },
@@ -110,7 +116,7 @@ export function useGridEditor(sessionId: string | null) {
     } finally {
       setLoading(false)
     }
-  }, [sessionId, ipaMode, syllableMode, ocrEnhance, ocrMaxCols, ocrMinConf])
+  }, [sessionId, ipaMode, syllableMode, ocrEnhance, ocrMaxCols, ocrMinConf, visionFusion, documentCategory])
 
   const loadGrid = useCallback(async () => {
     if (!sessionId) return
@@ -1030,6 +1036,10 @@ export function useGridEditor(sessionId: string | null) {
     setOcrMaxCols,
     ocrMinConf,
     setOcrMinConf,
+    visionFusion,
+    setVisionFusion,
+    documentCategory,
+    setDocumentCategory,
     rerunOcr,
   }
 }