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/Cargo.toml

@@ -17,6 +17,7 @@ tracing = { workspace = true }
 tracing-subscriber = { workspace = true }
 lettre = { workspace = true }
 gethostname = { workspace = true }
+chrono-tz = "0.8"
 toml = { workspace = true }
 async-trait = "0.1"
 reqwest = { version = "0.11", features = ["json", "blocking"] }