feat: OCR pipeline step 8 — validation view with image detection & generation

Replaces the stub StepGroundTruth with a full side-by-side Original vs
Reconstruction view. Adds VLM-based image region detection (qwen2.5vl),
mflux image generation proxy, sync scroll/zoom, manual region drawing,
and score/notes persistence.

New backend endpoints: detect-images, generate-image, validate, get validation.
New standalone mflux-service (scripts/mflux-service.py) for Metal GPU generation.
Dockerfile.base: adds fonts-liberation (Apache-2.0).

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

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

@@ -313,7 +313,7 @@ export default function OcrPipelinePage() {
       case 6:
         return <StepReconstruction sessionId={sessionId} onNext={handleNext} />
       case 7:
-        return <StepGroundTruth />
+        return <StepGroundTruth sessionId={sessionId} onNext={handleNext} />
       default:
         return null
     }