When an agent joins, we update last_seen_at in the database but don't set
last_heartbeat_db_update in the socket. This causes the heartbeat throttling
logic to be out of sync:
- On join: DB updated, but last_heartbeat_db_update not set (nil)
- First heartbeat: Updates DB again because last_heartbeat_db_update is nil
- Subsequent heartbeats: Check if >30s since last DB update
- Problem: last_heartbeat_db_update tracks the first heartbeat, not the join DB update
This manifests when Phoenix recompiles during development - the channel process
continues running with old socket assigns, and if last_heartbeat_db_update is
recent, heartbeats won't update the DB. After 10+ minutes without DB updates,
the agent appears offline even though it's still connected.
Fix: Set last_heartbeat_db_update when we update the DB on join, so the
throttling logic accurately tracks the most recent DB update.
Reviewed-on: graham/towerops-web#142
Graham McIntire
303c64ceae