Initial lip-sync service with command backend

docs/protocol.md

@@ -0,0 +1,21 @@
+# Protocol
+
+## Gradio endpoint
+
+- API name: `/process_single`
+- Audio input parameter: `audio_file`
+- Video input parameter: `video_file`
+- Output component: single `gr.Video`
+
+The function returns a video object compatible with Gradio client responses:
+
+```json
+{
+  "video": "D:/path/to/result.mp4",
+  "subtitles": null
+}
+```
+
+## Notes
+
+The upstream desktop app reads `result["video"]` from `scripts/digital_human_process.py`, so this service intentionally exposes one video output instead of multiple outputs. Adding extra Gradio outputs would change `client.predict` into a tuple and break compatibility.