Changed device_monitor.ex to always use ICMP ping for monitoring checks,
regardless of whether SNMP is enabled for the device.
Rationale:
- ICMP ping gives actual network latency (round-trip time)
- SNMP checks are for data collection (interfaces, sensors, etc.)
- Both should run independently for SNMP-enabled devices:
* SNMP polling: Collects device metrics (handled by poller_worker)
* ICMP monitoring: Measures latency and availability (handled by device_monitor)
This ensures latency graphs always show actual network response time,
not SNMP query response time.
Graham McIntire
c74e00fcb0