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>

klausur-service/Dockerfile.base

@@ -16,6 +16,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     tesseract-ocr-eng \
     libgl1 \
     libglib2.0-0 \
+    fonts-liberation \
     && rm -rf /var/lib/apt/lists/*
 
 # Python dependencies