Fresh clones errored out on `ddev start` because the compose file's
env_file: pointed at app/.env (gitignored). No env vars are actually
required by the code — STATE_DIR is the only one and has a default.
- Switch env_file to the path:+required:false form (Docker Compose
2.24+; bundled with DDEV) so missing app/.env is non-fatal.
- Commit app/.env.example as documentation for contributors who do
need overrides.