Discourse REST API: create topics and posts programmatically

Steps

1. Generate an API key from the target Discourse instance's admin panel (Admin > API), scoped either globally or to a specific user, per docs.discourse.org.
2. Send the key via the Api-Key and Api-Username headers — never as a URL query parameter, since Discourse's own docs warn query-string keys can leak via logs/traffic.
3. Create a new topic (with its first post) via POST /posts.json, supplying title, raw (Markdown body), and category (category id).
4. Create a reply in an existing topic using the same /posts.json endpoint but supplying topic_id instead of title/category.
5. Handle standard Discourse validation and rate limits (minimum post length, new-user posting limits) since API-created content is subject to the same site rules as UI-created content.