Refactor(plex): improve get users list flow

[arabcoders]

• Updated getUsers function in both BackendAdd.vue and edit.vue to support requesting tokens for a specific user by adding withTokens and targetUser parameters. This allows fetching tokens only for the selected user instead of all users.
• Added a "Generate Token" button to the Plex user selection UI in edit.vue, which triggers token generation for the currently selected user.
• Improved logic for assigning and updating tokens and user metadata.
• Added support for the target_user option in backend API endpoints and Plex user actions, so that token requests and responses are scoped to the specified user only.
• Improved error handling in Plex backend actions and ensured the correct user information is passed when requesting tokens.
• Added a test (test_get_users_list_target_token) to verify that tokens are only requested for the target user and not for all users.