chore(uptime-kuma): bump image to :2

The :1 image runs Node 18 (EOL), which gets no fix for the October 2026
Node.js security release. :2 migrates kuma.db in place on first start;
back up kuma_data per host before pulling (see plan/10-uptime-kuma.md).

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

docker-compose.yml

@@ -376,7 +376,7 @@ services:
   # Status monitoring + public status page. caddy proxies status. -> :3001.
   # Joins mcnet so monitors probe the stack by internal service name.
   uptime-kuma:
-    image: louislam/uptime-kuma:1
+    image: louislam/uptime-kuma:2
     container_name: uptime-kuma
     restart: unless-stopped
     volumes: