DEPOT logging: add Telemt fleet monitoring via Zabbix

Install and persist Grafana Zabbix plugin 6.5.0, provision the internal Zabbix datasource, and add a fleet dashboard modeled on client 02 Telemt panels.

Add an active-agent Zabbix 7 template plus a 30-second sanitized collector for active IP count/list, connections, traffic, and uptime. Active checks avoid inbound polling of client-private Docker hosts.

The exact Zabbix host names and required ZABBIX_API_TOKEN Portainer variable are documented. Compose validation passed on 10.0.0.6; the live plugin registered successfully under Grafana 13.1.0.

logging/syslog-ng.conf

@@ -0,0 +1,30 @@
+@version: 3.29
+@include "scl.conf"
+
+source s_local {
+    internal();
+};
+
+# default-network-drivers accepts the mixed/raw formats used by the DEPOT
+# fleet on the standard UDP/TCP syslog listeners.
+source s_network {
+    default-network-drivers();
+};
+
+# Alloy's syslog receiver accepts one explicit format per listener. Normalize
+# every accepted input to RFC5424 before relaying it over the private network.
+template t_alloy_rfc5424 {
+    template("<${PRI}>1 ${ISODATE} ${HOST} ${PROGRAM} ${PID} ${MSGID} - ${MSG}\n");
+};
+
+destination d_alloy {
+    syslog("alloy" transport("tcp") port("1514")
+        template(t_alloy_rfc5424)
+    );
+};
+
+log {
+    source(s_local);
+    source(s_network);
+    destination(d_alloy);
+};