refactor: simplify weather art creation process

- Moved weather art creation inside the transaction block
- Updated variable names for clarity
- Ensured the transaction block manages the creation and attachment of images more clearly

This refactor improves the readability and maintainability of the
`create_weather_art` method while preserving its original
functionality. The changes also ensure that the transaction
affects the creation and attachment operations coherently.

app/workers/generate_weather_art_worker.rb

@@ -40,8 +40,8 @@ class GenerateWeatherArtWorker
   def create_weather_art(weather_data, prompt, image_url)
     tempfile = nil
 
-    ActiveRecord::Base.transaction do
+    weather_art = ActiveRecord::Base.transaction do
-      weather_art = city.weather_arts.create!(
+      w = city.weather_arts.create!(
         weather_date: Date.today,
         prompt: prompt,
         **weather_data
@@ -49,12 +49,12 @@ class GenerateWeatherArtWorker
 
       tempfile = Down.download(image_url)
 
-      weather_art.image.attach(
+      w.image.attach(
         io: File.open(tempfile.path),
         filename: generate_filename,
         content_type: "image/png"
       )
-      weather_art
+      w
     end
     weather_art.webp_image.processed
     weather_art.preview_image.processed