Add backup directory creation and validation for WAL-G file backend

[gpas45]

Summary

This change adds automatic creation and validation of backup directories for WAL-G file-based backups, ensuring proper directory structure and permissions before backup operations begin.

Key Changes

• New ensure_backup_dir() function: Creates backup directories with proper ownership (postgres:postgres) and permissions (700) if they don't exist. Includes informative logging for all operations.
• Base directory validation: Added check to create/validate the base WALG_FILE_PREFIX_BASE directory during initial configuration setup.
• Per-cluster directory validation: For file backend configurations, validates that cluster-specific backup directories exist before processing each cluster. Skips clusters with missing directories and logs warnings.

Implementation Details

• Uses install -d command for atomic directory creation with proper ownership and permissions
• Provides clear INFO/OK/ERROR/WARN messages in Russian for operational visibility
• Gracefully handles missing directories by skipping affected clusters rather than failing the entire operation
• Only applies directory validation when using the file backend (BACKEND == "file")
• Returns appropriate exit codes (0 for success, 1 for failure) for error handling

https://claude.ai/code/session_01SiMQMpcCNttwku3vbdkhjn