Introduce traffic_spike alert type for monitoring system and per-master
traffic levels with configurable thresholds stored in database.
- Add AlertThresholdConfig model for persistent threshold configuration
- Implement GET/PUT /admin/alerts/thresholds endpoints for threshold management
- Add traffic spike detection in alert detector cron job:
- Global QPS monitoring across all masters
- Per-master RPM/TPM checks with minimum sample thresholds
- Per-master RPD/TPD checks for daily limits
- Use warning severity at threshold, critical at 2x threshold
- Include metric metadata (value, threshold, window) in alert details
- Update API documentation with new endpoints and alert type
