feat: add country filter in city admin

- Introduce a new filter for selecting cities by country in the
  ActiveAdmin interface.
- Update the ransackable attributes to include country_id for
  searchable functionality.

This change enhances the Admin UI by allowing easier access to
city data based on country, improving the user experience for
administrators.

app/admin/cities.rb

@@ -36,6 +36,7 @@ ActiveAdmin.register City do
 
   filter :name
   filter :active
+  filter :country, as: :select
 
   form do |f|
     f.inputs do

app/models/city.rb

@@ -45,7 +45,7 @@ class City < ApplicationRecord
   end
 
   def self.ransackable_attributes(auth_object = nil)
-    [ "active", "country", "created_at", "id", "id_value", "last_image_generation", "last_weather_fetch", "latitude", "longitude", "name", "priority", "region", "slug", "timezone", "updated_at" ]
+    [ "active", "country_id", "created_at", "id", "id_value", "last_image_generation", "last_weather_fetch", "latitude", "longitude", "name", "priority", "region", "slug", "timezone", "updated_at" ]
   end
 
   def last_weather_fetch