Port POST /user/export/request, POST /user/export/download (zip stream) and
GET /user/export (status) to v2. Extract the export-file loader and status
builder into pkg/models (GetUserDataExportFile, GetUserDataExportStatus) with
a shared ErrUserDataExportDoesNotExist, and refactor v1 onto them. The v2
download streams via the shared WriteFileDownload writer; local users confirm
with their password, external-provider users are passed through.
kolaente