Cameras S7b: plant regions on the reference frame, drift check, canopy-area metric, reference markers

S7 core landed 2026-09-07 (camera devices, five source kinds incl. motionEye satellite Pi and USB webcam on the brain, poller, retention, timelapse, zone-card thumbnail, Devices › Cameras drawer). Still open from plan-settings § S7: camera_regions (polygon per plant on the fixed frame, linked to a roster slot), corner-patch / marker drift check that marks regions STALE, canopy area in cm² labelled uncalibrated until a scale marker exists, camera_markers (scale · wall line · floor grid). Needs Pillow on the brain image for pixel work.