fix(monitor): quiet send-after-close race on WS disconnect

When a monitor subscriber disconnects mid-frame (the client portal closes its
stream on every tab switch via the Page Visibility guard), the loop could pull a
queued payload during the 1s wait and then send_json into an already-closing
socket -> "Unexpected ASGI message 'websocket.send' after ... websocket.close",
logged as a WARNING on every disconnect.

Re-check gone.done() after the queue wait and break before sending; treat the
residual send-after-close as expected (debug, not warning). No behavior change —
the connection was already closing as intended; this just stops the log spam.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/routers.py

@@ -284,10 +284,20 @@ async def monitor_stream(websocket: WebSocket, unit_id: str):
                 payload = await asyncio.wait_for(queue.get(), timeout=1.0)
             except asyncio.TimeoutError:
                 continue  # re-check gone.done()
+            if gone.done():
+                break  # client disconnected while we waited — don't send into a closing socket
             await websocket.send_json(payload)
     except WebSocketDisconnect:
         logger.info(f"Monitor subscriber disconnected for {unit_id}")
     except Exception as e:
+        # A frame that races the close (client vanished mid-send) surfaces as
+        # "Unexpected ASGI message 'websocket.send' after ... websocket.close".
+        # That's expected on disconnect (the portal closes the socket on every tab
+        # switch), not an error — log it quietly.
+        msg = str(e)
+        if "after sending" in msg or "websocket.close" in msg or "response already completed" in msg:
+            logger.debug(f"Monitor stream for {unit_id} closed mid-send (client gone)")
+        else:
             logger.warning(f"Monitor stream error for {unit_id}: {e}")
     finally:
         gone.cancel()