Add a friendly GET /api/ws response

A plain browser hit on the socket URL now returns a JSON hint instead of a
confusing 404/426. Companion to the Basic-auth bypass on /api/ws.

backend/app/api/routes.py

@@ -191,6 +191,15 @@ async def close_position(
     return engine.status()
 
 
+@router.get("/ws")
+def websocket_info() -> dict[str, str]:
+    """A plain browser hit on the socket URL gets a clear hint, not a 404/426."""
+    return {
+        "detail": "This endpoint is a WebSocket. Connect with a WebSocket client.",
+        "path": "/api/ws",
+    }
+
+
 @router.websocket("/ws")
 async def websocket_endpoint(websocket: WebSocket) -> None:
     runtime: Runtime = websocket.app.state.runtime