feat: implement asynchronous sandbox deletion with state machine

[fslongjin]

• Add SandboxDeletionService for background deletion processing with retry and force cleanup
• Extend sandbox model with deletion tracking fields (phase, attempts, force level, timestamps)
• Update DELETE API to return 202 Accepted and include deletion status in response
• Add include_terminating query parameter to list sandboxes under deletion
• Extend PVC mapping state to include 'deleting' for sandboxes in terminating state
• Update web UI to display deletion status and handle 202 responses
• Modify reconcile service to defer to deletion service for pending deletions
• Add comprehensive tests for deletion state transitions and persistence scenarios