Relay: log upstream events + forward error envelopes to device for diagnostics

Plan 3 smoke surfaced 'session_ended reason=error' on every wake with no
context. The relay now logs every upstream event type at INFO and forwards
any upstream 'error' event to the device as 'error code=upstream_<code>'
so it appears in the Pi's journal alongside the wake/session_ended pair.

Also extends the client's wakeword.predict with per-frame RMS + score
diagnostics and the bridge loop with a 5 s heartbeat for triage.

backend/Realtime/RealtimeSession.cs

@@ -65,11 +65,15 @@ public class RealtimeSession
         try
         {
             var enabled = _registry.EnabledFor(_cfg.EnabledTools).ToList();
+            _logger.LogInformation(
+                "relay opening session: model={Model} voice={Voice} tools=[{Tools}]",
+                _cfg.Model, _cfg.Voice, string.Join(",", enabled.Select(t => t.Name)));
             await _upstream.SendJsonAsync(RealtimeEvents.SessionUpdate(_cfg, enabled), ct);
 
             var ack = await WaitForUpstreamTypeAsync("session.updated", ct);
             if (ack is null)
             {
+                _logger.LogWarning("upstream closed before session.updated");
                 _endReason = EndReason.Error;
                 return;
             }
@@ -274,7 +278,25 @@ public class RealtimeSession
         {
             var evt = await _upstream.ReceiveJsonAsync(ct);
             if (evt is null) return null;
-            if ((string?)evt["type"] == type) return evt;
+            var evtType = (string?)evt["type"];
+            _logger.LogInformation("upstream evt={EvtType}", evtType);
+            if (evtType == "error")
+            {
+                var err = evt["error"] as JsonObject;
+                var code = (string?)err?["code"] ?? "unknown";
+                var msg = (string?)err?["message"] ?? evt.ToJsonString();
+                _logger.LogWarning("upstream error code={Code} message={Message}", code, msg);
+                await _output.WriteEnvelopeAsync(new JsonObject
+                {
+                    ["type"] = "error",
+                    ["code"] = "upstream_" + code,
+                    ["message"] = msg,
+                    ["fatal"] = false,
+                }, ct);
+                _endReason = EndReason.Error;
+                return null;
+            }
+            if (evtType == type) return evt;
         }
     }
 }