feat: make user email unique

Author: Backstage

src/Resources/UserResource/Schemas/UserForm.php

@@ -38,6 +38,7 @@ public static function configure(Schema $schema): Schema
                                             ->label(__('Email'))
                                             ->prefixIcon(fn (): BackedEnum => Heroicon::Envelope, true)
                                             ->email()
+                                            ->unique()
                                             ->required(),
 
                                         Select::make('roles')