Change the text or numbers on a live data slide

Pushes a JSON payload onto a dynamic slide's live data, validated against the template's data schema. When the slide is on a Push source, the data is stored on that source (shared by every slide on it; the slide stays hidden until the first push); otherwise it is stored on the slide and a re-render is queued. Data reaches a video-stream screen at most about once a minute: every push is stored at once, and the next render shows the newest payload, so pushing faster than that gains nothing. On HTML playback a live slide picks up each push without a render. Supports ?dryRun=true. One of the two endpoints a push-only isk_dp_ key may call.