[Feature Request] Optional WebSocket transport for event collection (self-hosted)

Describe the feature or enhancement We self-host Umami and track a mobile app through the API. Every event is its own POST to /api/send, and on longer sessions (game screens, lots of taps) that turns into a lot of requests. Would you consider an optional WebSocket endpoint for collecting events? I know /api/batch exists (#3275, from #1473) and it does help. But then the client has to keep a buffer and decide when to flush it, and if the app gets killed before that, those events are lost. That's pretty much the concern mikecao raised in #1473. With an open socket we could send each event right away and skip the per-request overhead. It would also help with two things people have asked about before: • current visitors uses a 5 min window, so it doesn't match who is actually connected (#1269, #2777) • single page sessions show 0s duration unless you send heartbeats (#3287). With a socket, connect/disconnect already tells you when the visit started and ended. I s…