Create a host (from a template or a full config)

Creates a host in the project and responds 201 with the new host's detail. Seed the host config one of two ways: pass template (a built-in template id, optional theme) to seed server-side from the live backend host catalog, falling back to the bundled SDK catalog snapshot if the live catalog is unavailable, OR pass config (a full host config v2). Exactly one of template or config is required.

Guest callers are denied (host creation is a write).