Add MQTT-based STT worker for VAD segments

stt-worker/README.md

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+# EVS STT Worker
+
+This worker subscribes to VAD events from MQTT, transcribes the referenced WAV files, and publishes text back to MQTT.
+
+Flow:
+- input topic: `evs/<device_id>/vad_segment`
+- reads: `wav_path` from JSON payload
+- output topic: `evs/<device_id>/transcript`
+- error topic: `evs/<device_id>/stt_error`
+
+Default model:
+- `STT_MODEL=small`
+- `STT_DEVICE=cpu`
+- `STT_COMPUTE_TYPE=int8`