Photos: the frame's own read-only address
Each frame has its own Google identity (a service account). You share a Drive folder, and optionally a calendar, with that address, with view-only access. The frame downloads the photos and shows them; it never uploads, changes or deletes anything, and it sees nothing that was not shared with it and approved on the frame. This part does not use a SweetFrame Google sign-in: nobody signs in or grants a permission here.