feat: per-device live monitor (fan-out) + alert evaluator (POC)

The piece the live-view + alerting work was building toward.

monitor.py — one DOD poll loop per device, broadcast to many subscribers:
- browser WebSockets (fixes the single-connection "second viewer sees
  nothing" contention — browsers no longer each open a device stream)
- the alert evaluator (can keep a feed running with no browser via
  /monitor/start, so alerting runs continuously)
- persistence (each snapshot written like the poller)
DOD-sourced, so the broadcast carries ln1/ln2 (which DRD cannot). All polls
go through the existing per-device lock + pool, so it serializes safely with
the background poller and on-demand commands.

alerts.py — pluggable POC evaluator: fires (logs) when ALERT_METRIC exceeds
ALERT_THRESHOLD_DB with an ALERT_COOLDOWN_SECONDS cooldown. The rule
(instantaneous vs sustained vs L10) is the single swap point; dispatch is a
server log for now (email/SMS later).

Endpoints:
- WS   /api/nl43/{unit_id}/monitor          subscribe to the shared feed
- POST /api/nl43/{unit_id}/monitor/start    keep feed alive w/o a browser
- POST /api/nl43/{unit_id}/monitor/stop     drop the keep-alive
- GET  /api/nl43/_monitor/status            running/subscribers/keepalive

WS endpoint races queue.get() against a disconnect watcher so an idle feed
still detects client drop and doesn't leak a subscription.

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

app/routers.py

@@ -246,6 +246,80 @@ async def system_resume():
     return {"status": "ok", "mode": "active", "message": "Polling resumed"}
 
 
+# ============================================================================
+# LIVE MONITOR (fan-out) — one DOD feed per device, broadcast to many clients
+# ============================================================================
+
+@router.websocket("/{unit_id}/monitor")
+async def monitor_stream(websocket: WebSocket, unit_id: str):
+    """Subscribe a browser to the device's shared 1 Hz DOD feed.
+
+    Any number of clients can attach without each opening its own device
+    connection (one poll loop per device, fanned out). Same JSON shape as the
+    DRD stream, but DOD-sourced so it includes ln1/ln2 (L1/L10).
+    """
+    await websocket.accept()
+    from app.monitor import monitor_manager
+
+    monitor = await monitor_manager.get(unit_id)
+    queue = await monitor.subscribe()
+    logger.info(f"Monitor subscriber attached for {unit_id} ({monitor.subscriber_count()} total)")
+
+    async def _watch_disconnect():
+        # Completes when the client disconnects, so an idle feed (no data) still
+        # detects the drop and we don't leak a subscription that keeps the device
+        # feed (and its connection) alive.
+        try:
+            while True:
+                msg = await websocket.receive()
+                if msg.get("type") == "websocket.disconnect":
+                    return
+        except Exception:
+            return
+
+    gone = asyncio.ensure_future(_watch_disconnect())
+    try:
+        while not gone.done():
+            try:
+                payload = await asyncio.wait_for(queue.get(), timeout=1.0)
+            except asyncio.TimeoutError:
+                continue  # re-check gone.done()
+            await websocket.send_json(payload)
+    except WebSocketDisconnect:
+        logger.info(f"Monitor subscriber disconnected for {unit_id}")
+    except Exception as e:
+        logger.warning(f"Monitor stream error for {unit_id}: {e}")
+    finally:
+        gone.cancel()
+        await monitor.unsubscribe(queue)
+
+
+@router.post("/{unit_id}/monitor/start")
+async def monitor_start(unit_id: str):
+    """Keep the device's feed running even with no browser attached, so alerting
+    evaluates continuously. Runtime-only (resets on restart)."""
+    from app.monitor import monitor_manager
+    monitor = await monitor_manager.get(unit_id)
+    await monitor.set_keepalive(True)
+    return {"status": "ok", "unit_id": unit_id, "running": monitor.running, "keepalive": True}
+
+
+@router.post("/{unit_id}/monitor/stop")
+async def monitor_stop(unit_id: str):
+    """Drop the keep-alive; the feed stops once no browser subscribers remain."""
+    from app.monitor import monitor_manager
+    monitor = await monitor_manager.get(unit_id)
+    await monitor.set_keepalive(False)
+    return {"status": "ok", "unit_id": unit_id, "keepalive": False}
+
+
+@router.get("/_monitor/status")
+async def monitor_status():
+    """Status of every device monitor (running, subscriber count, keep-alive)."""
+    from app.monitor import monitor_manager
+    return {"status": "ok", "monitors": monitor_manager.status()}
+
+
 # ============================================================================
 # GLOBAL POLLING STATUS ENDPOINT (must be before /{unit_id} routes)
 # ============================================================================