Settings > Sources: turn syslog reception on/off, choose UDP/TCP

- SyslogServer opens and closes the UDP/TCP listeners at runtime from the
  configuration saved in /data/syslog.json; a busy port no longer stops
  Logstream, the error is shown in Settings instead.
- Sources tab: syslog section with an on/off switch, UDP and TCP labels,
  the live listening state and the published port (SYSLOG_PORT, passed as
  SYSLOG_PUBLIC_PORT for display; the mapping stays in docker-compose).

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

main.go

@@ -120,16 +120,15 @@ func main() {
 		store.Enqueue(e)
 		hub.Publish(e)
 	}
-	if err := StartSyslog(ctx, cfg.syslogAddr, sink); err != nil {
-		log.Fatalf("syslog: %v", err)
-	}
+	// Listening errors (port already used…) are shown in Settings > Sources.
+	syslogSrv := NewSyslogServer(ctx, cfg.syslogAddr, getenv("SYSLOG_PUBLIC_PORT", ""), cfg.dataDir, sink)
 
 	static, err := fs.Sub(webFS, "web")
 	if err != nil {
 		log.Fatal(err)
 	}
 	mux := http.NewServeMux()
-	api := &API{store: store, hub: hub, tags: tags, rdns: rdns, allowPurge: cfg.allowPurge, exportMax: cfg.exportMax}
+	api := &API{store: store, hub: hub, tags: tags, rdns: rdns, allowPurge: cfg.allowPurge, exportMax: cfg.exportMax, syslog: syslogSrv}
 	if cfg.dockerLogs {
 		dm, err := NewDockerManager(cfg.dockerHost, cfg.dataDir, cfg.backfill, sink)
 		if err != nil {