Fix latency data not being captured in monitoring checks

The monitoring system was creating checks but hardcoding response_time_ms
to nil instead of using the actual ping latency. This caused the latency
graphs to have no data to display.

Changes:
- device_monitor.ex: Capture response time from ping/SNMP check result
- monitor_worker.ex: Use correct field name (response_time_ms) and status enum values (:success/:failure)

After this fix, new monitoring checks will include latency data and the
latency graphs will display properly on both device and site detail pages.

lib/towerops/monitoring/device_monitor.ex

@@ -84,17 +84,17 @@ defmodule Towerops.Monitoring.DeviceMonitor do
 
     now = DateTime.truncate(DateTime.utc_now(), :second)
 
-    status =
+    {status, response_time_ms} =
       case check_result do
-        {:ok, _time} -> :success
-        {:error, _reason} -> :failure
+        {:ok, time} -> {:success, time}
+        {:error, _reason} -> {:failure, nil}
       end
 
     # Save the check result
     case Monitoring.create_check(%{
            device_id: device_id,
            status: status,
-           response_time_ms: nil,
+           response_time_ms: response_time_ms,
            checked_at: now
          }) do
       {:ok, _check} ->

lib/towerops/workers/monitor_worker.ex

@@ -36,8 +36,8 @@ defmodule Towerops.Workers.MonitorWorker do
 
           Monitoring.create_check(%{
             device_id: device_id,
-            status: "up",
-            latency_ms: latency,
+            status: :success,
+            response_time_ms: latency,
             checked_at: DateTime.utc_now()
           })
 
@@ -48,8 +48,8 @@ defmodule Towerops.Workers.MonitorWorker do
 
           Monitoring.create_check(%{
             device_id: device_id,
-            status: "down",
-            latency_ms: nil,
+            status: :failure,
+            response_time_ms: nil,
             checked_at: DateTime.utc_now()
           })