feat: add CMS data cache layer

[glitchedmob]

Ports the CMS data access layer into SvelteKit server modules and adds the bounded public JSON cache that replaces Next revalidation.

• Moves CMS clients, data loaders, config, generated API types, and shared pure helpers into #lib paths with server-only code under src/lib/server.
• Adds an in-process allowlisted CMS JSON GET cache with 60-second freshness, stale-while-revalidate, cold request coalescing, hard stale expiry, bypass rules, and detached response bodies.
• Keeps Umbraco v18 type normalization and converts Chicago-local date parsing to plain Date instants for SvelteKit serialization.
• Updates OpenAPI generation output paths and root page policy loading to use the shared server config.