Send new data to a live data source

Pushes a JSON payload to a Push source, validated against the data schema of every template it feeds, and returns receivedAt. Every slide on the source updates; slides stay hidden until the first push and hide again after the source's hideAfterMinutes without data. Supports ?dryRun=true. Callable by a push-only isk_dp_ key bound to the source. 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.