feat: make gateway UDP port configurable via GATEWAY_UDP_PORT environment variable

Add GATEWAY_UDP_PORT environment variable with default value 51900 to replace hardcoded port 51820. Update DEFAULT_GATEWAY_ENDPOINT in .env.example to use new port. Add UDP port mapping to gateway service in docker-compose using GATEWAY_UDP_PORT. Add NEXAVPN_GATEWAY_LISTEN_PORT environment variable to gateway container. Update gateway-entrypoint.sh to use GATEWAY_LISTEN_PORT variable in bootstrap payload and WireGuard interface configuration.

deploy/docker-compose.yml

@@ -80,13 +80,16 @@ services:
       - /dev/net/tun:/dev/net/tun
     extra_hosts:
       - "host.docker.internal:host-gateway"
+    ports:
+      - "${GATEWAY_UDP_PORT:-51900}:${GATEWAY_UDP_PORT:-51900}/udp"
     environment:
       GATEWAY_BOOTSTRAP_TOKEN: ${GATEWAY_BOOTSTRAP_TOKEN:-nexavpn-gateway-bootstrap}
       NEXAVPN_GATEWAY_ID: ${NEXAVPN_GATEWAY_ID:-}
       NEXAVPN_GATEWAY_NAME: ${NEXAVPN_GATEWAY_NAME:-primary-gateway}
+      NEXAVPN_GATEWAY_LISTEN_PORT: ${GATEWAY_UDP_PORT:-51900}
       NEXAVPN_GATEWAY_SYNC_URL: ${NEXAVPN_GATEWAY_SYNC_URL:-http://host.docker.internal:8080/api/v1/gateway-agent}
       NEXAVPN_GATEWAY_BOOTSTRAP_URL: ${NEXAVPN_GATEWAY_BOOTSTRAP_URL:-http://host.docker.internal:8080/api/v1/gateway-agent/bootstrap}
-      DEFAULT_GATEWAY_ENDPOINT: ${DEFAULT_GATEWAY_ENDPOINT:-localhost:51820}
+      DEFAULT_GATEWAY_ENDPOINT: ${DEFAULT_GATEWAY_ENDPOINT:-localhost:${GATEWAY_UDP_PORT:-51900}}
       DEFAULT_VPN_CIDR: ${DEFAULT_VPN_CIDR:-100.96.0.0/24}
       NEXAVPN_API_TOKEN: ${NEXAVPN_API_TOKEN:-}
       NEXAVPN_GATEWAY_PRIVATE_KEY: ${NEXAVPN_GATEWAY_PRIVATE_KEY:-}