Fix zabbix agent hostnames for correct host assignment

- Container agent: rename from AgapHost to 'Zabbix server' so it monitors
  the Zabbix server container (was conflicting with the host agent)
- Enable passive listeners in container agent (remove ZBX_STARTAGENTS=0)
- Update 'Zabbix server' host interface from 127.0.0.1 to DNS zabbix-agent
  so the server can reach the agent over the backend Docker network

Host zabbix-agent2 (systemd) keeps hostname AgapHost for host monitoring.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

zabbix/docker-compose.yml

@@ -77,7 +77,6 @@ services:
       ZBX_HOSTNAME: ${AGENT_HOSTNAME}
       ZBX_SERVER_HOST: zabbix-server
       ZBX_SERVER_ACTIVE: zabbix-server
-      ZBX_STARTAGENTS: "0"
     volumes:
       - /etc/localtime:/etc/localtime:ro
     privileged: true