Fix remaining rubocop offenses

- Fix string literal style in doorkeeper.rb
- Add missing final newlines
- Remove trailing whitespace
- Fix array bracket spacing in migrations

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

db/migrate/20250618110736_add_owner_to_oauth_applications.rb

@@ -2,6 +2,6 @@ class AddOwnerToOauthApplications < ActiveRecord::Migration[7.2]
   def change
     add_column :oauth_applications, :owner_id, :uuid
     add_column :oauth_applications, :owner_type, :string
-    add_index :oauth_applications, [:owner_id, :owner_type]
+    add_index :oauth_applications, [ :owner_id, :owner_type ]
   end
 end