Add Node 24 | Remove Node 18 EOL | Tweak CI

Add: node:24-nginx

Tweak: missing matrix update for node:24-nginx

The weekly cron never refreshed npm because the buildx layer cache was
restored via a prefix restore-key on an unchanged master sha, so the
`npm i npm@latest` layer was always a cache hit and froze npm at 11.9.0.

Fold an ISO week stamp into the cache key + restore-key so the cache
rotates once a week: each new week is a cold build that re-resolves
npm@latest (and, with --pull, the base node image), while intra-week
pushes still hit the cache for fast incremental builds. The key-format
change also forces a one-time cold rebuild on merge, refreshing npm to
>=11.13 immediately.

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

ci: rotate buildx cache weekly so npm@latest actually refreshes