Always use ICMP ping for latency monitoring, not SNMP checks

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.

lib/towerops/monitoring/device_monitor.ex

@@ -12,10 +12,9 @@ defmodule Towerops.Monitoring.DeviceMonitor do
 
   # Allow dependency injection for testing
   @ping_module Application.compile_env(:towerops, :ping_module, Towerops.Monitoring.Ping)
-  @poller_module Application.compile_env(:towerops, :poller_module, Towerops.Snmp.Poller)
 
   # Suppress warnings for Mox modules that are defined at runtime during tests
-  @compile {:no_warn_undefined, [Towerops.Monitoring.PingMock, Towerops.Snmp.PollerMock]}
+  @compile {:no_warn_undefined, Towerops.Monitoring.PingMock}
 
   # Client API
 
@@ -73,14 +72,9 @@ defmodule Towerops.Monitoring.DeviceMonitor do
   defp perform_check(device_id) do
     device = Devices.get_device!(device_id)
 
-    # Use SNMP if enabled, otherwise fallback to ping
-    check_result =
-      if device.snmp_enabled do
-        client_opts = @poller_module.build_client_opts(device)
-        @poller_module.check_device(client_opts)
-      else
-        @ping_module.ping(device.ip_address)
-      end
+    # Always use ICMP ping for latency measurement
+    # SNMP polling is handled separately by the poller_worker
+    check_result = @ping_module.ping(device.ip_address)
 
     now = DateTime.truncate(DateTime.utc_now(), :second)