Adds a self-contained maintenance page that the SvelteKit handle hook
serves before the first CMS call when `isCmsAvailable()` returns
false. The probe hits `/api-docs/openapi.json` with a short timeout
and caches the result (10s healthy, 3s unhealthy) so we don't pay a
round-trip per request. `/maintenance` and `?cms_down=1` simulate
the page without taking the CMS down — useful for previewing the
copy.
The maintenance HTML is fully inline (no CMS calls, no layout, no
external CSS) so it survives even when every dependency is down.
Returned with 503 + Retry-After:60 + noindex headers.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Peter Meier