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/.env.example

@@ -0,0 +1,29 @@
+# Plan-B-Video — Windows video-render worker
+
+# NAS Redis 큐
+REDIS_URL=redis://192.168.45.54:6379
+
+# NAS internal webhook (video-lab port 18801)
+NAS_BASE_URL=http://192.168.45.54:18801
+INTERNAL_API_KEY=__copy_from_nas_dotenv__
+
+# Sora 2 (OpenAI)
+OPENAI_API_KEY=__paste_openai_key__
+
+# Veo 3.1 (Google Vertex AI)
+GOOGLE_PROJECT_ID=__paste_gcp_project_id__
+GOOGLE_LOCATION=us-central1
+GOOGLE_GCS_BUCKET=__paste_gcs_bucket_name__
+GOOGLE_APPLICATION_CREDENTIALS=/app/keys/gcp-sa.json
+
+# Kling (PiAPI gateway)
+PIAPI_API_KEY=__paste_piapi_key__
+
+# Seedance 2.0 (BytePlus)
+SEEDANCE_API_KEY=__paste_seedance_key__
+
+# NAS SMB mount 안의 video 디렉토리
+VIDEO_MEDIA_ROOT=/mnt/nas/webpage/data/video
+
+# nginx 서빙 prefix (NAS webhook payload용)
+VIDEO_MEDIA_URL_PREFIX=/media/video