Migrate to Docker: containerize for docker-server deployment

- Add Dockerfile + cron.js (daily 4pm UTC loop replacing EC2 cron)
- Add infra/docker-compose.yml and deploy-stack.sh for Portainer
- Support DATA_DIR env var in bot.js for persistent history volume
- Support PROMPTS_JSON env var in cron.js (no SSH needed for config)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,7 @@
+FROM node:18-alpine
+WORKDIR /app
+COPY package*.json ./
+RUN npm ci --omit=dev
+COPY bot.js scheduler.js cron.js ./
+ENV DATA_DIR=/data
+CMD ["node", "cron.js"]