Viewer retention curves for every video

You can now pull a viewer retention curve for any video in your catalog. The Data pipeline computes the percentage of viewers still watching at each point in playback, and the query API exposes the result as a normalized timeseries you can render directly in dashboards or BI tools. Highlights:

1. New metric — viewer retention is now a first-class metric in the query service, with the same filter, dimension, and time-range support as other metrics.
2. Per-video curves — request retention for a single video, a set of videos, or aggregate across a filter set.
3. Streaming-derived — retention is computed by the stream processing pipeline from playback heartbeat events, so it stays in sync with your actual viewing data with no separate instrumentation needed.