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/RealtimeSession.cs

@@ -128,9 +128,25 @@ public class RealtimeSession
             if (evt is null && ct.IsCancellationRequested) return;
             if (evt is null)
             {
-                // Upstream has gone silent. Surface that to the device every
-                // ~2s so we can tell "OpenAI is up but emitting nothing" from
-                // "the upstream WS was closed" — Coolify logs aren't reachable.
+                // If the upstream WS is closed, stop spinning — surface the
+                // close reason to the device and bail with EndReason=Error.
+                if (_upstream.IsClosed)
+                {
+                    var reason = _upstream.CloseReason ?? "upstream closed";
+                    _logger.LogWarning("upstream closed mid-session: {Reason}", reason);
+                    await _output.WriteEnvelopeAsync(new JsonObject
+                    {
+                        ["type"] = "error",
+                        ["code"] = "upstream_closed",
+                        ["message"] = reason,
+                        ["fatal"] = false,
+                    }, ct);
+                    _endReason = EndReason.Error;
+                    return;
+                }
+                // Surface "alive but silent" every ~2s so we can tell that
+                // from "the upstream WS was closed". Coolify logs aren't
+                // reachable from the dev sandbox.
                 if (++nullPolls % 8 == 0)
                 {
                     _logger.LogInformation("upstream silent ({Polls} x 250ms polls)", nullPolls);