Bước 8 — Build master narration timeline trên worker

Mục tiêu:
- Gom tất cả audio scene từ bước 7.
- Chèn pause/khoảng nghỉ theo workflow.
- Khoảng nghỉ trước heading scene: 1.0s.
- Khoảng nghỉ sau heading scene: 1.0s.
- Tạo audio/master_narration.wav.
- Tạo audio/master_narration_timeline.json.
- Tạo logs/step8_compose_tts_manifest.json.

Worker bắt buộc:
- Không dùng ffmpeg local để ghép master narration.
- Không compose audio-heavy trên host Erza.
- Phải gọi worker endpoint: http://192.168.1.104:8005/compose-tts

Input:
- script/scenes.json
- audio/scenes/scene_001.wav
- audio/scenes/scene_002.wav
- ...

Output:
- audio/master_narration.wav
- audio/master_narration_timeline.json
- logs/step8_compose_tts_manifest.json

Timeline item tối thiểu:
{
  "scene_id": "scene_001",
  "audio_path": "audio/scenes/scene_001.wav",
  "start": 0.0,
  "end": 8.74,
  "duration": 8.74
}

Compose endpoint request shape:
{
  "job_id": "<project_id>_step8_master_narration",
  "work_dir": "/data/video-pipeline/Youtube-Video-Maker/Project/<project_id>",
  "output_audio_path": "/data/video-pipeline/Youtube-Video-Maker/Project/<project_id>/audio/master_narration.wav",
  "tts_segments": [
    {
      "segment_index": 1,
      "start": 0.0,
      "end": 8.74,
      "audio_path": "/data/video-pipeline/Youtube-Video-Maker/Project/<project_id>/audio/scenes/scene_001.wav",
      "source_text": "...",
      "translated_text": "",
      "status": "ok"
    }
  ]
}

Rule quan trọng:
- File master_narration.wav là nguồn timing chính cho ASR/subtitle/split parts.
- Nhưng khi render từng part về sau, chuẩn mới là dùng exact part_XX.wav + exact part_XX.srt, không render bằng full master audio + offset nữa.

Checklist:
8. Build audio/master_narration.wav and scene timeline on worker using 192.168.1.104:8005/compose-tts from the per-scene TTS files generated in Step 7.