Server pages: wire the 503 maintenance page to restores, migrations and node work
Imported from GitHub issue El-SaMa/oma#162 by @El-SaMa.
The maint server page (https://oma.lanetti.fi/server-pages/maint) has no trigger yet.
Show it when we take a site offline (no WordPress page possible):
- Backsies restore in progress
- site migration final cutover
- node update / move
Idea: OMA sets a per-site marker at job start, nginx serves the page with 503 until the job removes it; Uptime Kuma gets 200 like WP maintenance does. Depends on the node rollout issue.