Add alt text for preview card thumbnails (#26184)

db/migrate/20230725213448_add_image_description_to_preview_cards.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+require Rails.root.join('lib', 'mastodon', 'migration_helpers')
+
+class AddImageDescriptionToPreviewCards < ActiveRecord::Migration[7.0]
+  include Mastodon::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  def up
+    safety_assured { add_column_with_default :preview_cards, :image_description, :string, default: '', allow_null: false }
+  end
+
+  def down
+    remove_column :preview_cards, :image_description
+  end
+end