Add a graphic to a brand kit

curl -X POST 'https://www.unitpost.com/api/v1/brand-kits/bk_123/graphics' \
  -H 'Authorization: Bearer YOUR_API_KEY' \
  -H 'User-Agent: my-app/1.0' \
  -H 'Content-Type: application/json' \
  -d '{ }'

{
  "object": "brand_kit_graphic",
  "brand_kit": {
    "object": "brand_kit",
    "id": "id_123",
    "name": "Example",
    "is_default": false,
    "website_url": "https://example.com",
    "description": "string",
    "tone": "NEUTRAL",
    "voice_notes": "string",
    "industry": "string",
    "core_pages": [
      {
        "url": "https://example.com",
        "title": "string"
      }
    ],
    "extracted": {
      "colors": [
        {
          "hex": "string",
          "label": "string"
        }
      ],
      "logo_urls": [
        "string"
      ],
      "background_urls": [
        "string"
      ],
      "fonts": [
        "string"
      ]
    },
    "graphics": [
      {
        "library_image_id": "library_image_123",
        "image_id": "image_123",
        "path": "string",
        "role": "logo",
        "label": "string"
      }
    ],
    "source": "string",
    "updated_at": "2026-01-01T00:00:00.000Z"
  },
  "image": {
    "object": "library_image",
    "id": "id_123",
    "path": "string",
    "url": "https://example.com",
    "filename": "string",
    "content_type": "string",
    "size": 1,
    "width": 0,
    "height": 0,
    "alt": "string",
    "created_at": "2026-01-01T00:00:00.000Z",
    "updated_at": "2026-01-01T00:00:00.000Z"
  }
}