fix(db): Add migration to change classic theme to akane-blue

* Add our prefix to avoid future conflicts with upstream

Author: lindwurm

db/migrate/20260328142000_akane_migrate_classic_themes.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+class AkaneMigrateClassicThemes < ActiveRecord::Migration[8.0]
+  disable_ddl_transaction!
+
+  # Dummy classes, to make migration possible across version changes
+  class User < ApplicationRecord; end
+
+  def up
+    User.where.not(settings: nil).find_each do |user|
+      settings = JSON.parse(user.attributes_before_type_cast['settings'])
+      next if settings.nil? || settings['theme'].blank?
+      next unless settings['theme'] == 'classic'
+
+      settings['theme'] = 'akane-blue'
+
+      user.update_column('settings', JSON.generate(settings))
+    end
+  end
+end