Start App Endpoint

Run the design doc 3.5 prechecks and start name, changing nothing on any failure.

Raises: PortalError: 404 app_not_found; 409 for every precheck refusal (app_busy, app_running, manifest_invalid, env_unbound, params_missing, params_invalid, venv_missing, platform_not_ready); 503 polkit_denied if systemd itself refuses the set-property/start verb; 500 start_failed (with a journal_tail) for any other error the unit start itself raises.