build: package the frontend Node runtime

[glitchedmob]

Package the existing SvelteKit Node server from tracked source and document the split CMS runtime.

• Add a Node 24 multi-stage Dockerfile with production dependencies and a restricted build context.
• Add npm install, type-check, snapshot freshness and build CI without deployment steps.
• Preserve the tracked SvelteKit 3 Vite configuration and map the public build origin to adapter-node 6.
• Document private runtime settings, request limits and builtin-only member schema limitations.