Change default cron schedule to every 5 hours
Reduce frequency from every 30 minutes to every 5 hours to avoid
excessive speedtest consumption and provide more reasonable default
for long-term monitoring.
Changes:
- .env.example: */30 * * * * → 0 */5 * * *
- docker-compose.yml: */30 * * * * → 0 */5 * * *
- README.md: Update examples and add every 5 hours to list
- AGENTS.md: Update default value in docs
Users who want more frequent testing can still customize via CRON_SCHEDULE.