Remove dev server deployment workflow

[jjoonleo]

Summary

• remove the dev EC2 deployment workflow so only production deploys run
• document that dev is CI/integration only under the one-EC2 private-RDS plan
• update EC2 deployment docs to point production deploys at main

Test plan

• git diff --check origin/dev...HEAD
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/deploy.yml"); YAML.load_file(".github/workflows/test.yml"); YAML.load_file("ontime-back/docker-compose.yml"); puts "yaml ok"'
• IMAGE_TAG=test BACKEND_IMAGE=ghcr.io/devkor-github/ontime-back docker compose -f ontime-back/docker-compose.yml config >/dev/null