fix(backup): handle 7z exit codes correctly to avoid false failures

[mrrobot47]

7z returns exit code 1 for non-fatal warnings (e.g., missing symlink targets like broken log file symlinks), but the archive is still created successfully. Previously, EE::exec() returned false for any non-zero exit code, causing false positive backup failures.

Changes:

• Switch from EE::exec() to EE::launch() to get actual exit codes
• Only fail on exit code >= 2 (fatal errors), allow exit code 1 (warnings)
• Add file existence checks as secondary validation
• Custom docker-compose backup logs warning instead of failing (optional)

7z exit code reference:

• 0: Success
• 1: Warning (non-fatal, archive created)
• 2: Fatal error
• 7: Command line error
• 8: Not enough memory

Updated functions:

• maybe_backup_custom_docker_compose()
• backup_site_dir()
• backup_wp_content_dir()
• backup_nginx_conf()
• backup_php_conf()
• backup_db()

[Copilot]

Pull request overview

This PR fixes false backup failures caused by improper handling of 7z exit codes. The tool returns exit code 1 for non-fatal warnings (like missing symlink targets), but the previous implementation using EE::exec() treated any non-zero exit code as a failure. The changes properly distinguish between warnings (exit code 1) and fatal errors (exit code >= 2).

Key Changes:

• Switched from EE::exec() to EE::launch() to access actual exit codes across all backup functions
• Updated error handling to only fail on exit code >= 2, treating exit code 1 as non-fatal
• Added file existence checks in most functions as secondary validation

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