// src/Backend/VanDaemon.Api/Hubs/TelemetryHub.cs
public class TelemetryHub : Hub
{
    public async Task SubscribeToTanks()
    {
        await Groups.AddToGroupAsync(Context.ConnectionId, "tanks");
    }

    public async Task SubscribeToControls()
    {
        await Groups.AddToGroupAsync(Context.ConnectionId, "controls");
    }

    public async Task SubscribeToAlerts()
    {
        await Groups.AddToGroupAsync(Context.ConnectionId, "alerts");
    }
}

// Inject IHubContext<TelemetryHub> in background service
await _hubContext.Clients.Group("tanks").SendAsync(
    "TankLevelUpdated", tankId, currentLevel, tankName, cancellationToken);

// In Blazor component
_hubConnection = new HubConnectionBuilder()
    .WithUrl($"{_apiBaseUrl}/hubs/telemetry")
    .WithAutomaticReconnect()
    .Build();

_hubConnection.On<Guid, double, string>("TankLevelUpdated", (id, level, name) =>
{
    // Update UI state
    InvokeAsync(StateHasChanged);
});

await _hubConnection.StartAsync();
await _hubConnection.InvokeAsync("SubscribeToTanks");

// Client subscribes only to what it needs
await _hubConnection.InvokeAsync("SubscribeToTanks");
await _hubConnection.InvokeAsync("SubscribeToControls");

// Server broadcasts to specific group
await _hubContext.Clients.Group("controls").SendAsync(
    "ControlStateChanged", controlId, newState, controlName);

_hubConnection.Closed += async (error) =>
{
    _isConnected = false;
    await InvokeAsync(StateHasChanged);
};

_hubConnection.Reconnected += async (connectionId) =>
{
    _isConnected = true;
    await _hubConnection.InvokeAsync("SubscribeToTanks");
    await InvokeAsync(StateHasChanged);
};