UniFi's Java process crashes during shutdown (Spring context race
condition) leaving mongod orphaned in the cgroup. The upstream module
sets KillSignal=SIGCONT so systemd won't interrupt the graceful
shutdown, but with the default KillMode=control-group this means
mongod also only gets SIGCONT (a no-op) and sits there until the
5-minute timeout triggers SIGKILL.
Switch to KillMode=mixed so the main Java process still gets the
harmless SIGCONT while mongod gets a proper SIGTERM for a clean
database shutdown.
