fix(slm): stop monitor proxy leaking CancelledError on stream stop

The /monitor WS proxy cancelled its sibling task on disconnect but then
`except Exception` failed to swallow the resulting CancelledError (a
BaseException), so stopping the stream raised "Exception in ASGI
application". It also only awaited the pending task, leaving the done
task's WebSocketDisconnect unretrieved ("Task exception was never
retrieved"). Await all tasks and catch (CancelledError, Exception).

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

backend/routers/slmm.py

@@ -269,10 +269,15 @@ async def proxy_websocket_monitor(websocket: WebSocket, unit_id: str):
         done, pending = await asyncio.wait(tasks, return_when=asyncio.FIRST_COMPLETED)
         for t in pending:
             t.cancel()
-        for t in pending:
+        # Await ALL tasks (the done one AND the cancelled one) and swallow both
+        # the expected WebSocketDisconnect and CancelledError. CancelledError is a
+        # BaseException, so a bare `except Exception` misses it — that's what leaked
+        # the traceback on stop; and awaiting only `pending` left the done task's
+        # exception unretrieved.
+        for t in tasks:
             try:
                 await t
-            except Exception:
+            except (asyncio.CancelledError, Exception):
                 pass
 
     except websockets.exceptions.WebSocketException as e: