feat: enforce trip membership and role authorization

backend/libs/trips/src/trip-roles.decorator.ts

@@ -0,0 +1,8 @@
+import { SetMetadata } from '@nestjs/common';
+
+export type TripRole = 'OWNER' | 'MEMBER';
+
+export const TRIP_ROLES_KEY = 'tripRoles';
+
+export const TripRoles = (...roles: TripRole[]) =>
+  SetMetadata(TRIP_ROLES_KEY, roles);