fix(init): only report systemd when it is actually running

[rvalitov]

Problem

detect_init_system() tested for the presence of the systemctl binary, not for systemd actually running as PID 1:

if command -v systemctl &>/dev/null; then
    echo "systemd"

A chroot, a container that pulled systemd in as a dependency, and a half-booted host all have the binary while every systemctl call fails. On those hosts the systemd branch of setup_autostart() was selected, then:

systemctl daemon-reload
systemctl enable mtproxymax.service 2>/dev/null
log_success "Auto-start enabled (systemd)"     # <-- unconditional

Both statuses were discarded, so the user was told "Auto-start enabled" when nothing had been enabled. The OpenRC branch directly below already verifies that the runlevel symlink actually landed, and returns 1 with a warning when it did not — the systemd branch did not.

Note systemctl --version is not a usable liveness probe either: it exits 0 with no bus at all.

Fix

• _systemd_is_running() gates detection on liveness: /run/systemd/system must exist and systemctl is-system-running must return something other than offline. The value is compared rather than the exit status, because degraded is still a booted system and exits non-zero.
• The systemd branch of setup_autostart() now checks the result of systemctl enable before claiming success, and returns 1 with a warning otherwise.

Checking enable's status (rather than is-enabled output) keeps this consistent with the existing systemctl stubs in tests/test_telegram_service_openrc.sh, which model exit statuses rather than stdout.

Testing

New tests/test_init_detection.sh drives a systemctl stub through three scenarios: binary present but not booted (must not be detected as systemd, must not report success), booted but enable failing (must not report success), and a working enable (must still report success). The last one guards against over-correcting into always failing.

Verified against real systemd — the integration test in the accompanying CI work still passes 14/14 on a container running systemd as PID 1, so the stricter detection does not reject a genuine systemd host.