Partially update a blog. Only fields included in the request body change; omitted fields are left untouched.

NOTE: slug cannot be changed here. It is not a field on UpdateBlogPayload and is stripped defensively in the service, because the slug is the blog's public URL — changing it drops SEO ranking and breaks inbound links. Rename a slug only via a deliberate direct DB change.

{
  "content_html": "<p>Updated content</p>",
  "content_markdown": "Updated content",
  "hero_image_url": "https://example.com/hero.png",
  "is_active": true,
  "is_draft": false,
  "language_code": "en",
  "meta_description": "Updated meta description",
  "related_blog_ids": [
    1,
    2
  ],
  "title": "Updated blog title",
  "written_by": "Author Name"
}