PATCH /api/brand-logos/{id} — JSON:API field-level partial update.

Per https://jsonapi.org/format/#crud-updating-resource-attributes, only
the top-level attributes/relationships present in the request body are
replaced; missing top-level fields are preserved. A client can patch
just name without disturbing width, alt_text, the image
relationship, etc.

A present top-level field is replaced wholesale (no recursive merge
into nested structures), so sending "alt_text": null clears it.

The image relationship is optional on PATCH — omit to keep the
existing image; include with a non-null data to repoint at a
different image. A logo must have an image, so the JSON:API
"unlink" form ("image": {"data": null}) is not supported and is
treated the same as omitting the relationship (existing image
preserved).