fix(devices): make upsert_devices/1 work with Repo.insert_all

Three related bugs prevented upsert_devices/1 from ever succeeding in
production:

- The JSON map keys ("identifier", "vendor", etc.) were passed through
  to Repo.insert_all, which only accepts atom field names.
- updated_at was a %DateTime{} but the Devices schema uses :naive_datetime.
- inserted_at was never set, violating the NOT NULL constraint.

Fix by whitelisting known string keys and converting them to atoms via
String.to_existing_atom/1 (safe against atom exhaustion), using
NaiveDateTime.utc_now/1, and setting both timestamps explicitly.

lib/aprsme/device_identification.ex

@@ -135,6 +135,10 @@ defmodule Aprsme.DeviceIdentification do
     end
   end
 
+  # The Devices schema uses :naive_datetime for its timestamps, so insert_all
+  # needs a NaiveDateTime, not a DateTime.
+  defp now_for_insert, do: NaiveDateTime.utc_now(:second)
+
   def fetch_and_upsert_devices do
     case CircuitBreaker.call(:aprs_foundation_api, &fetch_devices_from_url/0, 15_000) do
       {:ok, result} -> result
@@ -159,7 +163,7 @@ defmodule Aprsme.DeviceIdentification do
     tocalls = Map.get(json, "tocalls", %{})
     mice = Map.get(json, "mice", %{})
     micelegacy = Map.get(json, "micelegacy", %{})
-    now = DateTime.utc_now()
+    now = now_for_insert()
 
     all_devices =
       Enum.flat_map([tocalls, mice, micelegacy], fn group ->
@@ -177,15 +181,24 @@ defmodule Aprsme.DeviceIdentification do
     :ok
   end
 
+  # Whitelist of JSON keys that map directly to Devices schema fields.
+  # Using an explicit list avoids String.to_atom on untrusted input.
+  @device_fields ~w(identifier class model vendor os contact features)
+
   defp process_device_attrs(attrs, identifier, now) do
     attrs
     |> Map.put("identifier", identifier)
-    |> Map.update("features", nil, fn f ->
-      if is_list(f), do: f, else: [f]
-    end)
-    |> Map.put("updated_at", now)
+    |> Map.update("features", nil, &normalize_features/1)
+    |> Map.take(@device_fields)
+    |> Map.new(fn {k, v} -> {String.to_existing_atom(k), v} end)
+    |> Map.put(:inserted_at, now)
+    |> Map.put(:updated_at, now)
   end
 
+  defp normalize_features(features) when is_list(features), do: features
+  defp normalize_features(nil), do: nil
+  defp normalize_features(other), do: [other]
+
   @doc """
   Looks up a device by identifier, using ? as a single-character wildcard.
   Returns the device struct if found, or nil.