feat(status): Uptime Kuma monitoring at status.${BASE_DOMAIN}

Add docker-compose.status.yml (louislam/uptime-kuma) layered on the caddy
ingress, joining mcnet so it probes the stack by internal service name and
the public vhosts end-to-end. Host port bound to 127.0.0.1:3001 only; public
access via nginx -> caddy.

- caddy/conf.d/50-status.caddy: status. -> uptime-kuma:3001
- nginx caddy-front template: status. TLS vhost + websocket upgrade headers
  for Kuma's live UI; status. added to the HTTP->HTTPS redirect list
- status added to LE_SUBDOMAINS defaults (.env.example, issue-letsencrypt.sh)
- README: Status monitoring section + recommended monitor list (incl. native
  Minecraft-protocol ping)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

caddy/conf.d/50-status.caddy

@@ -0,0 +1,5 @@
+# Status toggle — Uptime Kuma monitoring + public status page
+# (docker-compose.status.yml). Mounted only when the status override is present.
+http://status.{$BASE_DOMAIN} {
+	reverse_proxy uptime-kuma:3001
+}