Avoid systemctl enable loop in installer by adding robust service helpers

[arun-lmnas]

Motivation

• The installer was getting stuck in a loop when trying to systemctl enable services in environments where systemd is not running or behaves differently (containers), so boot-time service enabling must be conditional.
• The intent is to make the script safely start services during installation while avoiding attempts to enable services at boot in container-style environments where that triggers repeated systemd-sysv-install activity.

Description

• Add ENABLE_BOOT_SERVICES (default auto) and running_in_container/should_enable_boot_services helpers so boot-time enabling is skipped automatically in containers.
• Add systemd_available, service_ctl, start_service, restart_service, and enable_service helpers to use systemctl only when available and fall back to service otherwise with timeout protections.
• Replace direct systemctl enable --now and systemctl restart calls with the new start_service/enable_service/restart_service flows for mariadb and the common services stage.
• Update recovery/instructions to prefer service status/restart commands where appropriate so they work in non-systemd environments.

Testing

• Ran bash -n install-frappe-press.sh to validate shell syntax and it succeeded.
• Ran git diff --check to ensure there are no whitespace/patch problems and it succeeded.
• Checked for shellcheck but it is not installed in this environment so automated linting was not performed.

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Codex Task