Retain transcript messages and add MQTT publish diagnostics

stt-worker/README.md

@@ -7,6 +7,9 @@ Flow:
 - reads: `wav_path` from JSON payload
 - output topic: `evs/<device_id>/transcript`
 - error topic: `evs/<device_id>/stt_error`
+- default retain:
+  - transcript retained (`STT_TRANSCRIPT_RETAIN=true`)
+  - error non-retained (`STT_ERROR_RETAIN=false`)
 
 Default model:
 - `STT_MODEL=small`