scrypted: snapshot-then-stream for sub-second first paint on wake

ffmpeg cold-start + RTSP connect + first-keyframe-wait puts 0.5–3
seconds of dead air between a tap/event and the first stream frame
landing on the panel. Most of that is unavoidable for the live
stream, but Scrypted can almost always produce a snapshot in
50–300ms via camera.takePicture() (often a cache hit).

New pushSnapshot path:
- Fires in parallel with the main stream spawn — does NOT delay the
  stream path even if takePicture is slow.
- takePicture → quick one-shot ffmpeg with the same transpose+scale
  filter chain → POST /frame, all under a 2s hard cap.
- Uses the shared X-Frame-Seq counter: if a stream frame beats the
  snapshot to the firmware decoder (unlikely on cold start, possible
  on warm reconnect), the firmware silently stale-drops the snapshot.
  Logs "beaten by stream frame" when that happens so we can see it.
- Errors are swallowed silently — a camera that doesn't support
  snapshots just falls through to the normal stream-only path.

Net effect: the panel shows the camera near-instantly on every wake,
then the snapshot gets replaced by the first ffmpeg frame whenever
it lands.

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