Add PUT /api/v2/user/settings/avatar, the first multipart/form-data file
upload on the Huma-backed v2 API. Reuses v1's byte-level mime validation
(mimetype.DetectReader) and storage (upload.StoreAvatarFile), modeling the
request as a huma.MultipartFormFiles input so it renders as multipart/form-data
in the OpenAPI spec instead of being read off the raw echo context.
Flips the user's avatar provider to "upload" on success. Authenticated (JWT).