Add GET /api/v2/avatar/{username}, the v2 reference for a binary response
modeled in the OpenAPI spec. Reuses the v1 avatar provider logic (provider
lookup, size clamp to config.ServiceMaxAvatarSize, runtime content-type) and
returns raw image bytes via Huma's []byte body + dynamic Content-Type header
idiom, advertised in the spec as application/octet-stream.
The endpoint is authenticated under the global security like every other v2
route (an anonymous request gets a 401); it is not public.
kolaente