Increase history retention to 20, run scrapers every 4 hours

- generate_status.py: raise --keep default from 5 to 20 entries
- build/crontab: change schedule from 06:00/18:00 to every 4 hours (*/4)
  covers 6 runs/day ≈ 3.3 days of history at default retention

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

build/crontab

@@ -1 +1 @@
-0 6,18 * * * cd /app && bash /app/run_all.sh >> /proc/1/fd/1 2>> /proc/1/fd/2
+0 */4 * * * cd /app && bash /app/run_all.sh >> /proc/1/fd/1 2>> /proc/1/fd/2