Add cron service to run load_from_static 4x daily

[Copilot]

Implements a cron job to automatically sync data from Life-USTC/static cache into the database every 6 hours (00:00, 06:00, 12:00, 18:00 UTC).

Changes

• src/crontab - Cron schedule: 0 0,6,12,18 * * *
• src/Dockerfile.cron - Dedicated image with cron daemon, git (for repo cloning), and Django dependencies. Entrypoint pipes environment vars to /etc/environment so cron jobs inherit DB credentials and Django settings.
• docker-compose.yml / docker-compose.prod.yml - Added cron service that shares database network with web service
• .github/workflows/docker-build-push-cron.yml - CI/CD to build and push ghcr.io/life-ustc/server-cron:latest

Usage

# Development
docker-compose up

# Production  
docker-compose -f docker-compose.prod.yml up

# View logs
docker-compose logs -f cron

The cron container runs indefinitely with tail -f /var/log/cron.log, outputting job execution logs to stdout.

Original prompt

set python manage.py load_from_static to be a cronjob running 4 times per day

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