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>

README.md

@@ -124,6 +124,6 @@ See `docs/plugins/` for a full plugin development guide.
 | `docs/plugins/writing-a-plugin.md` | Step-by-step plugin development guide |
 | `docs/plugins/traefik.md` | Traefik plugin: config, metrics, alert rule examples |
 | `docs/plugins/prometheus.md` | Prometheus plugin: scraping any /metrics endpoint, how series are stored, the Fabled Archives recipe |
-| `docs/plugins/syslog.md` | Syslog plugin: sources, patterns and alerts, publishing port 5514 (host mode under Swarm) |
+| `docs/plugins/syslog.md` | Syslog plugin: sources, patterns and alerts, publishing port 5514 (ingress or host mode under Swarm) |
 | `docs/plugins/prometheus-widgets.md` | Prometheus widget templates: writing, sharing and copying one in, every block, the format reference |
 | `docs/reference/code-map.md` | Where every key function, model, and route lives in the codebase |