fix: support older systemd hardening settings

tests/release-scripts.test.sh

@@ -469,6 +469,11 @@ test_linux_service_preflight_requires_systemd() (
   NO_SERVICE=1 PATH="$tmp/bin" validate_service_manager
 )
 
+test_systemd_unit_uses_compatible_protection() (
+  grep -Fqx 'ProtectSystem=full' <(sed -n '/^    cat > \/etc\/systemd\/system\/kaidi-erp.service/,/^    systemctl daemon-reload/p' "$PROJECT_ROOT/install.sh")
+  ! grep -Fq 'ProtectSystem=strict' "$PROJECT_ROOT/install.sh"
+)
+
 test_no_service_install_disables_online_update() (
   local tmp
   tmp="$(mktemp -d "${TMPDIR:-/tmp}/erp-no-service-test.XXXXXX")" || return 1
@@ -520,6 +525,7 @@ run_test 'update helper rejects a concurrent process' test_update_lock_rejects_c
 run_test 'installer moves database setup to the web wizard' test_installer_moves_database_setup_to_web_wizard
 run_test 'installer requires an explicit Gitea URL' test_installer_requires_explicit_gitea_url
 run_test 'Linux production install requires systemd' test_linux_service_preflight_requires_systemd
+run_test 'systemd unit uses compatible protection' test_systemd_unit_uses_compatible_protection
 run_test 'no-service install disables online update' test_no_service_install_disables_online_update
 run_test 'release workflow uses the scoped Gitea job token' test_release_workflow_uses_scoped_job_token
 run_test 'installer starts correctly when piped to bash' test_installer_runs_when_piped_to_bash