Changes to Get an HLS CMAF playlist for a video. Returns a master playlist when `track` is omitted, or a single-track media playlist when `track` is supplied. The playlist references each segment + per-track init segment via getVideoBlob, addressed by content hash. The `uri` is an AT-URI pointing at a playable record — today only place.stream.video records are supported, but the surface is collection-agnostic so other record types can join later.