Design a new slide template from a description or HTML

Creates a workspace-owned template. title is required, plus exactly one of code mode (html + css; every {{field}} in the HTML becomes a data field) or AI mode (prompt describing the design + sampleJson, an example of the data it shows). "dataMode": "push" makes a template whose data an outside system pushes (see POST /v1/sources/{id}/data). Example: {"title": "Queue", "html": "<h1>Now serving {{number}}</h1>", "css": "h1{font-size:12vh}", "dataMode": "push"}. Try a design with POST /v1/templates/preview first. Supports ?dryRun=true. Needs a plan that includes AI Studio, except one code-mode push template on any plan (the free live data slide); otherwise EntitlementRequired with upgradeUrl.