feat(syslog): devices through Swarm's ingress told apart by name; the send-to address is a setting (#6096)

Through ingress every device arrives from Docker's own address, so a
source there is keyed by the hostname its lines carry (new column
sender, unique with address). The set-up help no longer guesses the
address from the web hostname: it shows the plugin's send_to setting,
covers ingress and host mode, ASUSTOR ADM, and both syslog formats.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

plugins/syslog/plugin.yaml

@@ -11,8 +11,11 @@ tags:
 
 # Sources are added from the Syslog page (/plugins/syslog/), where a device
 # that starts sending is offered. Steward runs as an unprivileged user, so
-# the port is above 1024: point devices at it, or map 514 to it.
+# the port is above 1024: point devices at it, or map 514 to it. send_to is
+# the address the page's set-up help tells devices to send to (a swarm node's
+# IP, or an address shared over the nodes when the port is on Swarm's ingress).
 config:
+  send_to: ""
   port: 5514
   udp: true
   tcp: true