FEATURE: Add dark mode option for category backgrounds (#24003)

Adds a new upload field for a dark mode category background that will be used as an alternative when Discourse is using a dark mode theme.

app/models/category.rb

@@ -29,6 +29,7 @@ class Category < ActiveRecord::Base
   belongs_to :uploaded_logo, class_name: "Upload"
   belongs_to :uploaded_logo_dark, class_name: "Upload"
   belongs_to :uploaded_background, class_name: "Upload"
+  belongs_to :uploaded_background_dark, class_name: "Upload"
 
   has_many :topics
   has_many :category_users
@@ -115,8 +116,13 @@ class Category < ActiveRecord::Base
 
   after_save do
     if saved_change_to_uploaded_logo_id? || saved_change_to_uploaded_logo_dark_id? ||
-         saved_change_to_uploaded_background_id?
-      upload_ids = [self.uploaded_logo_id, self.uploaded_logo_dark_id, self.uploaded_background_id]
+         saved_change_to_uploaded_background_id? || saved_change_to_uploaded_background_dark_id?
+      upload_ids = [
+        self.uploaded_logo_id,
+        self.uploaded_logo_dark_id,
+        self.uploaded_background_id,
+        self.uploaded_background_dark_id,
+      ]
       UploadReference.ensure_exist!(upload_ids: upload_ids, target: self)
     end
   end
@@ -1185,6 +1191,7 @@ end
 #  allow_unlimited_owner_edits_on_first_post :boolean          default(FALSE), not null
 #  default_slow_mode_seconds                 :integer
 #  uploaded_logo_dark_id                     :integer
+#  uploaded_background_dark_id               :integer
 #
 # Indexes
 #

app/models/category_list.rb

@@ -16,6 +16,7 @@ class CategoryList
   def self.included_associations
     [
       :uploaded_background,
+      :uploaded_background_dark,
       :uploaded_logo,
       :uploaded_logo_dark,
       :topic_only_relative_url,

app/models/site.rb

@@ -81,6 +81,7 @@ class Site
                   :uploaded_logo,
                   :uploaded_logo_dark,
                   :uploaded_background,
+                  :uploaded_background_dark,
                   :tags,
                   :tag_groups,
                   :form_templates,