feat(repo): add CLA mechanism for external contributors

Introduce a Contributor License Agreement (CLA) so external contributions
can be licensed under AGPL-3.0 and any other terms (incl. closed-source),
keeping the AGPL-3.0 codebase usable in closed-source projects.

- CLA.md: authoritative English CLA (ICLA + employer authorization, v1.0)
- CLA.zh.md: non-binding Chinese reference translation
- CONTRIBUTING.md: bilingual contributing guide, points to CLA
- .github/workflows/cla.yml: self-hosted cla-assistant-action that records
  signatures into cla-signatures/version-1.json; exempts maintainers & bots
  via allowlist; skips when CLA_BOT_TOKEN is unset
- .github/PULL_REQUEST_TEMPLATE.md: guides contributors to sign
- README.{md,en.md,ja.md}: add License & contributing footer
- app/terms: note CLA requirement in the IP section

Enforcement requires repo-level setup (PAT secret + branch protection)
documented in cla.yml; not covered by this commit.

README.en.md

@@ -221,3 +221,11 @@ See the [Bring-your-own voice Key guide](docs/xiaomi-tts-key.md) for how to obta
 ## Star history
 
 [![Star History Chart](https://api.star-history.com/svg?repos=zonghaoyuan/infiplot&type=Date)](https://star-history.com/#zonghaoyuan/infiplot&Date)
+
+---
+
+## License & contributing
+
+This project is open-sourced under [AGPL-3.0](https://www.gnu.org/licenses/agpl-3.0.html).
+
+Contributions are welcome! External contributors must sign our Contributor License Agreement (CLA) once before a PR can be merged — see [CONTRIBUTING.md](CONTRIBUTING.md) and [CLA.md](CLA.md).