feat(video-render): scaffold + nas_client webhook adapter (SP-7)

Dockerfile (python:3.12-slim), requirements (openai + google-cloud-storage + httpx + redis).
.env.example: OPENAI/GOOGLE/PIAPI/SEEDANCE keys + VIDEO_MEDIA_ROOT.
nas_client.webhook_update_task: call-time os.getenv (테스트 격리), respx mock 5 tests.
Plan-B-Video Phase 2.

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

services/video-render/Dockerfile

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+FROM python:3.12-slim-bookworm
+ENV PYTHONUNBUFFERED=1
+
+WORKDIR /app
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    ca-certificates \
+ && rm -rf /var/lib/apt/lists/*
+
+COPY requirements.txt .
+RUN pip install --no-cache-dir --timeout 600 --retries 5 -r requirements.txt
+
+COPY . .
+
+EXPOSE 8000
+CMD ["python", "-m", "uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8000", "--workers", "1"]