The MJPEG fan-out broadcaster from #1089 only solved viewer-side
concurrency. Obico polling (every 5s) and the manual /camera/snapshot
endpoint kept opening their own fresh RTSP sockets, which X1/H2/P2
firmwares tolerated but X2D firmware 01.01.00.00 enforces strict
single-connection on — every poll kicked the live stream.
Add try_get_active_buffered_frame(printer_id): returns the broadcaster's
last buffered frame when a viewer is connected, None otherwise. Obico
and /camera/snapshot consult it before opening a fresh socket. When no
viewer is active they fall through to the existing fresh-capture path.
plate_detection and layer_timelapse intentionally not converted.
maziggy
c097140e4c