[codex] Prepare EC2 backend deployment

[jjoonleo]

Summary

• Prepare EC2 deployment for the Spring backend container.
• Generate deploy-only runtime config and secret files from GitHub Secrets.
• Mount Firebase and Apple secret files at runtime instead of packaging them into the jar.
• Keep the database external through SPRING_DATASOURCE_URL and Flyway DB secrets.
• Move deployment docs under ontime-back/docs/deployment/ec2.md.

Deploy behavior

This PR targets deploy. After it is merged, the push to deploy should trigger the Deploy workflow.

Validation

• ./gradlew build -x test
• docker compose config
• YAML parse check for .github/workflows/deploy.yml