fix: guard device cache refresh when stopped

lib/aprsme/device_cache.ex

@@ -46,7 +46,10 @@ defmodule Aprsme.DeviceCache do
   Refreshes the device cache from the database.
   """
   def refresh_cache do
-    GenServer.call(__MODULE__, :refresh_cache)
+    case Process.whereis(__MODULE__) do
+      nil -> :ok
+      _pid -> GenServer.call(__MODULE__, :refresh_cache)
+    end
   end
 
   # Server callbacks

test/aprsme/device_cache_test.exs

@@ -0,0 +1,29 @@
+defmodule Aprsme.DeviceCacheTest do
+  use ExUnit.Case, async: false
+
+  alias Aprsme.DeviceCache
+
+  setup do
+    pid = Process.whereis(DeviceCache)
+
+    if pid do
+      :erlang.unregister(DeviceCache)
+    end
+
+    on_exit(fn ->
+      if pid && !Process.whereis(DeviceCache) do
+        true = Process.register(pid, DeviceCache)
+      end
+    end)
+
+    :ok
+  end
+
+  test "refresh_cache/0 returns :ok when the cache server is not running" do
+    assert DeviceCache.refresh_cache() == :ok
+  end
+
+  test "lookup_device/1 returns nil on cache miss without crashing when server is not running" do
+    assert DeviceCache.lookup_device("TEST-DEVICE") == nil
+  end
+end