feat(cron): add automatic alert detector for anomaly monitoring

Implement AlertDetector background task that runs every minute to detect
and create alerts for various anomalies:

- Rate limit detection: monitors masters hitting rate limits
- Error spike detection: flags keys with >= 10% error rate
- Quota exceeded: warns when key quota usage >= 90%
- Provider down: alerts when API keys have >= 50% failure rate

Includes fingerprint-based deduplication with 5-minute cooldown to
prevent duplicate alerts for the same issue.

cmd/server/main.go

@@ -207,6 +207,13 @@ func main() {
 	defer cancelToken()
 	go tokenRefresher.Start(tokenCtx)
 
+	// Alert Detector
+	alertDetectorConfig := cron.DefaultAlertDetectorConfig()
+	alertDetector := cron.NewAlertDetector(db, logDB, rdb, service.NewStatsService(rdb), alertDetectorConfig, logger)
+	alertDetectorCtx, cancelAlertDetector := context.WithCancel(context.Background())
+	defer cancelAlertDetector()
+	go alertDetector.Start(alertDetectorCtx)
+
 	adminService, err := service.NewAdminService()
 	if err != nil {
 		fatal(logger, "failed to create admin service", "err", err)