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>

scraper_stats.py

@@ -2,12 +2,14 @@
 from __future__ import annotations
 
 import json
+import os
 from pathlib import Path
 
 HERE = Path(__file__).parent
+DATA_DIR = Path(os.environ.get("DATA_DIR", HERE))
 
 
 def write_stats(filename: str, stats: dict) -> None:
-    """Write scraper run stats dict to a JSON file next to this module."""
-    path = HERE / filename
+    """Write scraper run stats dict to the data directory."""
+    path = DATA_DIR / filename
     path.write_text(json.dumps(stats, ensure_ascii=False, indent=2), encoding="utf-8")