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/requirements.txt

@@ -0,0 +1,9 @@
+fastapi==0.115.6
+uvicorn[standard]==0.34.0
+playwright==1.48.0
+jinja2>=3.1.4
+Pillow>=10
+redis>=5.0
+httpx>=0.27
+pytest>=8.0
+pytest-asyncio>=0.24