feat: support multiple stop IDs in alerts config

Author: digitalcora

lib/config/alerts.ex

@@ -1,14 +1,17 @@
 defmodule ScreensConfig.Alerts do
   @moduledoc false
 
-  @type t :: %__MODULE__{stop_id: String.t()}
+  @type t :: %__MODULE__{stop_ids: [String.t()]}
 
-  @enforce_keys [:stop_id]
+  @enforce_keys [:stop_ids]
   defstruct @enforce_keys
 
   use ScreensConfig.Struct
 
-  defp value_from_json(_, value), do: value
+  # Migrate from the previous format which only supported a singular `stop_id`.
+  defp migrate_json(%{"stop_id" => stop_id} = json), do: Map.put(json, "stop_ids", [stop_id])
+  defp migrate_json(other), do: other
 
+  defp value_from_json(_, value), do: value
   defp value_to_json(_, value), do: value
 end