A company runs a content management system (CMS) on a fleet of EC2 instances behind an Application Load Balancer, spread across three Availability Zones in an Auto Scaling group. All instances must read and write to a common set of uploaded media files and user documents with a POSIX file system interface. The current design stores these files on an EBS volume attached to a single instance, which has become a single point of failure and cannot be shared. Which storage solution best provides shared, highly available access for the entire fleet with the least operational overhead?