feature(fleet): add batch vehicle lookup by ids

Add a new /api/fleet/vehicle/by-ids endpoint in the fleet service to retrieve multiple vehicles in a single request. Expose getVehiclesByIds(List ids) in VehicleService and implement it using repository findAllById with existing VehicleResponse mapping.

This enables user-service and circuit-service to fetch vehicle details for many drivers at once and avoids N+1 calls to the fleet API, significantly reducing latency in driver assignment and planning flows.