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>

.gitignore

@@ -1,5 +1,6 @@
 node_modules/
 .env
+infra/.env
 .state.json
 history.json
 history-*.json