GET /printers/:id/snapshot asks the farm's gateway for one JPEG frame
(camera_snapshot_request) and returns it no-store. Concurrent viewers
share one read and a frame under 800 ms old is reused, because the
printer's camera serves one client. /stream answers kind "snapshot" for
these printers; printers.camera_snapshot is set from telemetry.
scripts/e2e-camera-snapshot.sh reads real frames from the P1P through
the real proxy and backend.
