feat(auth): implement role-based access control and user management features

backend/src/auth/decorators/roles.decorator.ts

@@ -0,0 +1,4 @@
+import { SetMetadata } from '@nestjs/common';
+
+export const ROLES_KEY = 'roles';
+export const Roles = (...roles: string[]) => SetMetadata(ROLES_KEY, roles);