Fix DATA_DIR usage in stats/history paths, set env in Dockerfile, add validation docs

- scraper_stats.py: respect DATA_DIR env var when writing stats_*.json files
- generate_status.py: read stats files and write history from DATA_DIR instead of HERE
- build/Dockerfile: set DATA_DIR=/app/data as default env var
- docs/validation.md: end-to-end Docker validation recipe

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

build/Dockerfile

@@ -5,6 +5,7 @@ RUN apk add --no-cache curl bash tzdata \
     && echo "Europe/Prague" > /etc/timezone
 
 ENV PYTHONUNBUFFERED=1
+ENV DATA_DIR=/app/data
 
 WORKDIR /app