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/20250618104425_add_source_to_api_keys.rb

@@ -1,6 +1,6 @@
 class AddSourceToApiKeys < ActiveRecord::Migration[7.2]
   def change
     add_column :api_keys, :source, :string, default: "web"
-    add_index :api_keys, [:user_id, :source]
+    add_index :api_keys, [ :user_id, :source ]
   end
 end

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

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