Diagnostics: surface upstream WS close reason + break the null-spin loop

The Pi journal showed 170 000 'upstream silent NxN' messages within one
second — i.e. ReceiveJsonAsync was returning null in zero time, so the
upstream WS to OpenAI was closed. The relay was spinning until idle
watchdog (30 s) fired. Add IRealtimeUpstream.IsClosed + CloseReason,
populated by OpenAIRealtimeUpstream from the close status / WS exception,
and have PumpAsync stop on close and forward
  error code=upstream_closed message=close status=<X> desc=<Y>
to the device. Closes the spin AND gives us the OpenAI close reason.

backend/Realtime/OpenAIRealtimeUpstream.cs

@@ -7,6 +7,10 @@ namespace backend.Realtime;
 public class OpenAIRealtimeUpstream : IRealtimeUpstream
 {
     private readonly ClientWebSocket _ws = new();
+    private string? _closeReason;
+
+    public bool IsClosed => _ws.State != WebSocketState.Open && _ws.State != WebSocketState.Connecting;
+    public string? CloseReason => _closeReason;
 
     public async Task ConnectAsync(string apiKey, string model, CancellationToken ct)
     {
@@ -31,11 +35,27 @@ public class OpenAIRealtimeUpstream : IRealtimeUpstream
         while (true)
         {
             WebSocketReceiveResult result;
-            try { result = await _ws.ReceiveAsync(buffer, ct); }
-            catch (WebSocketException) { return null; }
-            catch (OperationCanceledException) { return null; }
+            try
+            {
+                result = await _ws.ReceiveAsync(buffer, ct);
+            }
+            catch (WebSocketException exc)
+            {
+                _closeReason ??= $"WebSocketException:{exc.WebSocketErrorCode}:{exc.Message}";
+                return null;
+            }
+            catch (OperationCanceledException)
+            {
+                return null;
+            }
 
-            if (result.MessageType == WebSocketMessageType.Close) return null;
+            if (result.MessageType == WebSocketMessageType.Close)
+            {
+                var status = _ws.CloseStatus?.ToString() ?? "<none>";
+                var desc = _ws.CloseStatusDescription ?? "<no description>";
+                _closeReason ??= $"close status={status} desc={desc}";
+                return null;
+            }
             stream.Write(buffer, 0, result.Count);
             if (result.EndOfMessage)
             {