Implement comprehensive monitoring improvements

- Add full email notifications with lettre and Stockholm timezone
- Add status persistence to prevent notification spam on restart
- Change nginx monitoring to check backend proxy_pass URLs instead of frontend domains
- Increase nginx site timeout to 10 seconds for backend health checks
- Fix cache intervals: disk (5min), backup (10min), systemd (30s), cpu/memory (5s)
- Remove rate limiting for immediate notifications on all status changes
- Store metric status in /var/lib/cm-dashboard/last-status.json

agent/src/metrics/mod.rs

@@ -182,11 +182,13 @@ impl MetricCollectionManager {
         for collector in &self.collectors {
             let collector_name = collector.name();
 
-            // Determine cache interval for this collector type - ALL REALTIME FOR FAST UPDATES
+            // Determine cache interval for this collector type based on data volatility
             let cache_interval_secs = match collector_name {
-                "cpu" | "memory" | "disk" | "systemd" => 2, // All realtime for fast updates
-                "backup" => 10, // Backup metrics every 10 seconds for testing
-                _ => 2,         // All realtime for fast updates
+                "cpu" | "memory" => 5,    // Fast updates for volatile metrics
+                "systemd" => 30,          // Service status changes less frequently
+                "disk" => 300,            // SMART data changes very slowly (5 minutes)
+                "backup" => 600,          // Backup status changes rarely (10 minutes)
+                _ => 30,                  // Default: moderate frequency
             };
 
             let should_collect =