Port PUT /projects/{project}/backgrounds/upload to the Huma-backed v2 API. The
multipart handler reuses handler.ValidateAndSaveBackgroundUpload (shared with
v1), checks project write access explicitly, and is gated on the upload provider
config flag. Adds webtests covering the happy path, auth/permission failures,
non-image rejection, the disabled-provider case and the multipart spec shape.
kolaente