Add an RWX PVC backend for shared data

Description

Implement sharedDataBackend=rwx-pvc for /var/task, model cache data, and configured shared-data paths. Provision ReadWriteMany PVCs from an explicitly selected StorageClass or an explicit opt-in to the default StorageClass. Preserve SMB behavior when RWX mode is not selected.

Expose PVC phase, bound volume and access-mode details, retry state, timeout state, and useful failure messages through shared-storage status.

Definition of Done

• Tests cover explicit and default StorageClass selection, missing or ambiguous classes, sizing, PVC creation, binding, and SMB fallback.
• Workload mount paths remain compatible.
• Healthy RWX mode does not require an NVCF-managed SMB server for shared data.