feat: monitor heartbeat + background poller skips active-monitored units

- Heartbeat: if nothing has been broadcast in MONITOR_HEARTBEAT_S (default
  25s) — e.g. device offline and silent — send a non-cached keepalive frame
  so a reverse proxy (NPM) doesn't drop the idle WS. New subscribers still
  get the last real frame, not a heartbeat.
- Poller-skip: the 60s background poller now skips any unit with a running
  monitor (MonitorManager.is_active). The monitor already polls it ~1Hz and
  keeps the status cache fresh, so the background poll was redundant and just
  added load/lock-contention on the device's single connection (and churn,
  which matters for the cellular wedge). Trade-off: the FTP start-time sync
  (only in the poller) doesn't run while a unit is actively monitored — fine,
  since reports take the authoritative start time from the FTP .rnd data.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/background_poller.py

@@ -178,10 +178,19 @@ class BackgroundPoller:
             now = datetime.utcnow()
             polled_count = 0
 
+            from app.monitor import monitor_manager
+
             for cfg in configs:
                 if not self._running:
                     break
 
+                # Skip units with an active live monitor: it polls them at ~1Hz and
+                # keeps the status cache fresh, so a redundant background poll would just
+                # add load/lock-contention on the device's single connection.
+                if monitor_manager.is_active(cfg.unit_id):
+                    self._logger.debug(f"Skipping {cfg.unit_id} — live monitor active")
+                    continue
+
                 # Get current status
                 status = db.query(NL43Status).filter_by(unit_id=cfg.unit_id).first()