Auto-resolve device_up recovery alerts on creation

device_up alerts are informational recovery events, not ongoing issues.
Previously they were created without resolved_at, inflating the
unresolved alert count. Now they're born resolved.

Includes a data migration to fix existing stale device_up alerts.

lib/towerops/workers/device_monitor_worker.ex

@@ -237,6 +237,7 @@ defmodule Towerops.Workers.DeviceMonitorWorker do
         device_id: device.id,
         alert_type: :device_up,
         triggered_at: now,
+        resolved_at: now,
         message: recovery_message
       })
 

priv/repo/migrations/20260214204700_resolve_device_up_alerts.exs

@@ -0,0 +1,17 @@
+defmodule Towerops.Repo.Migrations.ResolveDeviceUpAlerts do
+  use Ecto.Migration
+
+  def up do
+    # device_up alerts are recovery notifications and should always be resolved.
+    # Previously they were created without resolved_at, leaving them "unresolved" forever.
+    execute """
+    UPDATE alerts
+    SET resolved_at = triggered_at
+    WHERE alert_type = 'device_up' AND resolved_at IS NULL
+    """
+  end
+
+  def down do
+    :ok
+  end
+end