refactor: tidy up code style and remove unused fields

- Added a space in the array passed to `add_index` in the migration
- Removed unused columns `last_weather_fetch` and `last_image_generation` from the `cities` table
- Ensured consistent code style in the `GenerateWeatherArtWorker` and added required newline at the end of files

These changes improve code readability and maintainability while ensuring that
no unnecessary fields exist in the database schema.

app/workers/batch_generate_weather_arts_worker.rb

@@ -33,4 +33,4 @@ class BatchGenerateWeatherArtsWorker
         .where("latest_arts.last_generation_time IS NULL OR latest_arts.last_generation_time < ?", cutoff_time)
         .order(:priority)
   end
-end
+end

app/workers/generate_weather_art_worker.rb

@@ -50,7 +50,7 @@ class GenerateWeatherArtWorker
       weather_art.image.attach(
         io: File.open(tempfile.path),
         filename: generate_filename,
-        content_type: 'image/png'
+        content_type: "image/png"
       )
 
       weather_art
@@ -65,4 +65,4 @@ class GenerateWeatherArtWorker
   def generate_filename
     "#{city.country.name}-#{city.name.parameterize}-#{Time.current.strftime('%Y%m%d-%H%M%S')}.png"
   end
-end
+end

db/migrate/20250123155234_remove_last_fetch_fields_from_cities.rb

@@ -3,6 +3,6 @@ class RemoveLastFetchFieldsFromCities < ActiveRecord::Migration[8.0]
     remove_column :cities, :last_weather_fetch
     remove_column :cities, :last_image_generation
 
-    add_index :weather_arts, [:city_id, :weather_date]
+    add_index :weather_arts, [ :city_id, :weather_date ]
   end
 end