fix: use actual memory usage excluding cache/buffers in monitoring

- Replace v.Used with (Total - Available) calculation
- Provides accurate memory usage for monitoring alerts
- Prevents false alerts caused by Linux cache/buffers
- Aligns with 'free -h' available memory metric

apps/monitoring/monitoring/monitor.go

@@ -130,7 +130,8 @@ func GetServerMetrics() database.ServerMetric {
 	}
 
 	memTotalGB := float64(v.Total) / 1024 / 1024 / 1024
-	memUsedGB := float64(v.Used) / 1024 / 1024 / 1024
+	memAvailableGB := float64(v.Available) / 1024 / 1024 / 1024
+	memUsedGB := memTotalGB - memAvailableGB
 	memUsedPercent := (memUsedGB / memTotalGB) * 100
 
 	var networkIn, networkOut float64