Move the Cache-Control: no-cache header into the shared WriteFileDownload
so every export and attachment download carries it, and add it to the
standalone v1 export download writer too. Downloads must never be cached.
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