Creates a brand, optionally already named and with its own image

The multipart sibling of POST /v2/settings/brands. The request is multipart/form-data with four optional parts: 'brandLabel', 'agencyId', 'organizationId' and a 'picture' file part - the same picture contract as POST /v2/media and the profile-picture endpoints. 'organizationId' (from GET /v2/organizations) creates the brand in that organization, under its owner's account and brand limit; it requires manageBrands there and cannot be combined with 'agencyId'. Every part may be omitted, and omitting all of them creates the same empty brand the JSON variant creates. The image is validated and normalised server-side to an 80x80 PNG, centre-cropped, before the brand is created, so an invalid image creates nothing. Accepted formats PNG, JPEG and WebP, decided from the actual bytes. Maximum upload size 1 MB, and neither side of the source image may exceed 2000 pixels - both are rejections, not automatic downscales. Once the brand exists the image is best-effort: if storing it fails the brand is still created and the response carries no image URL. Without a name the brand shows as an empty brand until its first account is connected; without an image it falls back to the image derived from that account.