feat(services/insta-render): Dockerfile + requirements + env.example (SP-3 scaffold)

Windows WSL2 Docker용 Chromium 워커 컨테이너 기본 골격.
다음 task에서 main.py, worker.py, card_renderer.py 작성.

Plan-B-Insta Phase 2 시작.

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

services/insta-render/Dockerfile

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+FROM python:3.12-slim-bookworm
+ENV PYTHONUNBUFFERED=1
+
+WORKDIR /app
+
+# Korean fonts + Chromium runtime deps (Debian 12 / bookworm)
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    fonts-noto-cjk fonts-noto-cjk-extra \
+    libnss3 libnspr4 libdbus-1-3 libatk1.0-0 libatk-bridge2.0-0 \
+    libcups2 libdrm2 libxkbcommon0 libxcomposite1 libxdamage1 \
+    libxfixes3 libxrandr2 libgbm1 libxshmfence1 libpango-1.0-0 \
+    libcairo2 libasound2 libatspi2.0-0 \
+ && rm -rf /var/lib/apt/lists/*
+
+COPY requirements.txt .
+RUN pip install --no-cache-dir --timeout 600 --retries 5 -r requirements.txt
+RUN playwright install chromium
+
+COPY . .
+
+EXPOSE 8000
+CMD ["python", "-m", "uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8000", "--workers", "1"]