fix(music-render): F6 ReliableQueue 적용 (F6 part 3)

- worker.py: poll_once 신설, BLPOP → ReliableQueue.dequeue/ack/fail + startup recovery
- 12 job_type dispatch table 보존 (기존 13 tests 그대로 PASS)
- Dockerfile: build context=services/, _shared 포함, PYTHONPATH=/app
- docker-compose.yml: music-render build context 갱신

dispatch 자체 unhandled exception 발생 시 fail(raw, payload)로 retry/dead-letter.
provider 함수가 webhook("failed")를 잡고 있는 정상 케이스는 ack (멱등 webhook).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

services/music-render/Dockerfile

@@ -8,10 +8,13 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     ca-certificates \
  && rm -rf /var/lib/apt/lists/*
 
-COPY requirements.txt .
+COPY music-render/requirements.txt /app/
 RUN pip install --no-cache-dir --timeout 600 --retries 5 -r requirements.txt
 
-COPY . .
+# F6: 공통 ReliableQueue 모듈 (services/_shared)
+COPY _shared /app/_shared
+COPY music-render/. /app/
+ENV PYTHONPATH=/app
 
 EXPOSE 8000
 CMD ["python", "-m", "uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8000", "--workers", "1"]