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.

client/wakeword.py

@@ -39,9 +39,14 @@ class WakewordDetector:
     def predict(self, frame_16k_int16: np.ndarray) -> bool:
         scores = self._model.predict(frame_16k_int16)
         score = float(scores.get(WAKEWORD, 0.0))
+        rms = float(np.sqrt(np.mean(frame_16k_int16.astype(np.float32) ** 2)))
         now = time.monotonic()
+        # Diagnostic: log any non-trivial score so we can see what's getting
+        # close to firing.
+        if score >= 0.1:
+            _log.info("score=%.3f rms=%.0f", score, rms)
         if score >= self._threshold and (now - self._last_fired) >= self._cooldown_s:
             self._last_fired = now
-            _log.info("wakeword fired score=%.3f", score)
+            _log.info("wakeword fired score=%.3f rms=%.0f", score, rms)
             return True
         return False