When admin opens an entity preview, find a kiosk whose active layout
references the camera (new repo.listKiosksRenderingCamera). Probe each
candidate's LAN snapshot endpoint with a 4s timeout. On success, stream
the bytes back with x-bf-snapshot-source: kiosk:<id>. Falls through to
the existing server-direct ffmpeg/gst pull only when no kiosk is reachable
or has the camera in its active layout.
Kiosk side adds /local/snapshot/:camera_id?key=<local_key>. Spawns a
one-shot gst-launch (rtspsrc → decodebin → jpegenc ! filesink
num-buffers=1) on a blocking worker so axum's reactor stays free.
Prefers sub stream for snapshots to keep bandwidth low. Single-frame
pipeline tears down after the first JPEG.
LAN IP picking extracted to shared/kiosk-lan.ts so route handler and
KioskLocalPanel agree on which interface to talk to (the previously-
duplicated logic in admin-pages stays for now since it also renders the
interface list).
Why a parallel pipeline instead of teeing the warm one: cross-thread
gtk4paintablesink → appsink sample extraction is non-trivial. A 1-frame
parallel pull is cheap when the kiosk's RTSP session to that camera is
already known to work (precondition: it's in the active layout).
