feat: disconnect clients immediately on certificate revocation (#1432)

## Summary

Adds immediate client disconnect when a certificate is revoked, via
OpenVPN management interface.

Previously, revoked clients stayed connected until they voluntarily
disconnected or the server restarted.

Fixes #1199

## Changes

- Enable management interface (Unix socket at
`/var/run/openvpn/server.sock`)
- Add `disconnectClient()` function to send `kill` command on revoke
- Add `socat` dependency for socket communication

test/Dockerfile.server

@@ -15,6 +15,7 @@ ENV ENABLE_NFTABLES=${ENABLE_NFTABLES}
 
 # Install basic dependencies based on the OS
 # dnsutils/bind-utils provides dig for DNS testing with Unbound
+# Note: socat is installed by openvpn-install.sh during OpenVPN installation
 RUN if command -v apt-get >/dev/null; then \
         apt-get update && apt-get install -y --no-install-recommends \
             iproute2 iptables curl procps systemd systemd-sysv dnsutils jq \