feat(assignment): use UUID-based vehicle lookup to resolve driver seat capacity

resolveDriverSeatCapacity was calling getDriversWithVehicleByIds which reads vehicle data from a static local JSON file instead of the live fleet service, resulting in seatCapacity always being null. Switch to getDriversWithVehicleByUuids which fetches real-time vehicle data from the fleet service, ensuring seatCount is properly resolved for same-establishment overlap validation.