Generate URL base

Returns a single template including its `available_fields` — the keys you can pass in the `data` object of `POST /api/v1/renders`. **Field schema by engine:** - **Typst / Canvas** — keys extracted from the template source: `{ key, type: "text", example }` where `example` is the placeholder value in the source. - **DOCX** — the template's authored field definitions with richer metadata: `{ key, label, type, required }` where `type` is one of `text`, `date`, `number`, or `array`. Also returns `url_base` — the template's on-demand URL token, or `null` if not generated yet. See [On-demand URLs](/docs/api/on-demand-urls).