Add backend api and engine

engine/README.md

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+# py-active-call-cc
+
+Python Active-Call: real-time audio streaming with WebSocket and WebRTC.
+
+This repo contains a Python 3.11+ codebase for building low-latency voice
+pipelines (capture, stream, and process audio) using WebRTC and WebSockets.
+It is currently in an early, experimental stage.
+
+# Usage
+
+启动
+
+```
+uvicorn app.main:app --reload --host 0.0.0.0 --port 8000
+```
+
+测试
+
+```
+python examples/test_websocket.py
+```
+
+```
+python mic_client.py
+```