chore: add LICENSE, CHANGELOG, and release workflow for v1.0.0

- Apache-2.0 LICENSE file
- CHANGELOG.md with 1.0.0 release notes from recent PRs
- GitHub Actions release workflow (triggers on v* tags)
- Align backend version to 1.0.0

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>