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/20250618110104_create_mobile_devices.rb

@@ -12,6 +12,6 @@ class CreateMobileDevices < ActiveRecord::Migration[7.2]
       t.timestamps
     end
     add_index :mobile_devices, :device_id, unique: true
-    add_index :mobile_devices, [:user_id, :device_id], unique: true
+    add_index :mobile_devices, [ :user_id, :device_id ], unique: true
   end
 end