Get the lab-view occupancy status for a project

Return the project's sites, cyclers, and channels with occupancy state.

Each channel carries a derived free / occupied / stale state (there is no live telemetry; state comes from linked measurements' end_time and update recency). Includes per-cycler and project-level occupancy counts. Each occupied/stale channel's measurement carries a watched flag for the requesting user, so the "My Channels" filter needs no extra round-trip.