One round-trip the polling client uses to refresh its view.

Side effects:

• Pings the caller's presence so they appear as active to other clients without a separate request.
• Reconciles the caller's lock state from the optional held_locks query string (format: block_id:token,block_id:token): tokens still claimed are heartbeated; locks held by the caller that aren't in the list are released immediately.

All DB calls are synchronous psycopg under the hood; each runs on the threadpool via asyncio.to_thread so the event-loop worker isn't parked, and asyncio.gather collapses them to one wall-clock round-trip.