fix: stamp backend/compass versions on install

[riley0227]

Summary

• Fresh installs never wrote .backend-version or compass/.version files
• The upgrade checker reads those files, finds 0.0.0, and always shows "update available" nag even on a brand new install
• Install script now resolves latest backend and compass versions from GitHub releases and writes them after install

Release checklist (if merging to main)

• ix-cli/package.json version bumped
• After merge: tag pushed
• If backend changes: ix-memory-layer tagged and released first
• If docker-compose.standalone.yml changed: verified curl | sh install works