Detect device offline status using MQTT Last Will and application-level heartbeats

Steps

1. Configure LWT in every device's CONNECT packet: set willTopic to devices/<id>/status, willMessage to {"online":false}, willQoS to 1, and willRetain to true so new subscribers immediately see the last known status
2. On successful connection, immediately publish {"online":true} with retained=true to the same devices/<id>/status topic to overwrite the LWT from any previous session; this establishes a clear online indicator
3. Implement an application-level heartbeat: have each device publish a small message (e.g., {"ts":<epoch_ms>}) to devices/<id>/heartbeat on a fixed interval (e.g., every 60 seconds) independent of telemetry traffic
4. On the backend, maintain a last-seen timestamp per device updated on every heartbeat receipt; run a periodic job (e.g., every 30 seconds) that compares now() against last-seen and marks devices as offline if they have not been heard within 2-3 heartbeat intervals
5. Combine both signals: use LWT for fast detection of clean disconnects and the heartbeat timeout for detecting silent failures (device hangs, network black-hole) where TCP stays open but the device stops sending
6. Emit offline events to an alerting or logging system when a device transitions to offline; include the last-seen timestamp and the reason (LWT vs heartbeat timeout) to distinguish clean disconnects from crash or network failure scenarios