Ping periódico desde la app. Actualiza last_seen_at en la licencia y guarda en heartbeats.

lmcp-api#274 (colibird-ai/.github#189) — pacing contract with the clients:

• the response carries an integer next_beat_s: how long the client should wait before its next liveness ping. Default 900 (HEARTBEAT_FLOOR_S), LMCP_HEARTBEAT_NEXT_BEAT_S overrides it, always inside [300, 3600]. A client that does not know the field ignores it and keeps beating every 5 minutes, which every reader still supports.
• a beat that changed the state fingerprint or carries work (tool calls, errors, slow calls, journeys) is stored as a heartbeats row; a pure liveness ping only refreshes machines and the hourly liveness counter. A client sends at once when its state or batch changes, and otherwise on next_beat_s (15 minutes floor).