feat(ocr-pipeline): add Step 5 word recognition (grid from columns × rows)

Backend: build_word_grid() intersects column regions with content rows,
OCRs each cell with language-specific Tesseract, and returns vocabulary
entries with percent-based bounding boxes. New endpoints: POST /words,
GET /image/words-overlay, ground-truth save/retrieve for words.
Frontend: StepWordRecognition with overview + step-through labeling modes,
goToStep callback for row correction feedback loop.
MkDocs: OCR Pipeline documentation added.

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

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

@@ -112,6 +112,16 @@ export default function OcrPipelinePage() {
     }
   }
 
+  const goToStep = (step: number) => {
+    setCurrentStep(step)
+    setSteps((prev) =>
+      prev.map((s, i) => ({
+        ...s,
+        status: i < step ? 'completed' : i === step ? 'active' : 'pending',
+      })),
+    )
+  }
+
   const handleNext = () => {
     if (currentStep < steps.length - 1) {
       setSteps((prev) =>
@@ -161,7 +171,7 @@ export default function OcrPipelinePage() {
       case 3:
         return <StepRowDetection sessionId={sessionId} onNext={handleNext} />
       case 4:
-        return <StepWordRecognition />
+        return <StepWordRecognition sessionId={sessionId} onNext={handleNext} goToStep={goToStep} />
       case 5:
         return <StepCoordinates />
       case 6: