fix!: Enforce `NOT NULL` for required database columns

Changed

• allow protected scope to $configuration, $superglobals and getCurrentReturnUrl() in UserModel, so that a child class (in an app) can access it

Fixed

• fix!: Enforce NOT NULL for required authentication and group database columns across supported Phinx versions. (Required NOT NULL constraints cannot be safely reverted because prior nullability depends on the Phinx version.)
• Validate email option types before passing them to string-only validators.