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.

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,27 @@
+## Summary
+
+<!-- Briefly describe what this PR changes, and why. Link any related issues. -->
+
+## CLA
+
+- [ ] I have read and signed the [Contributor License Agreement (CLA)](https://github.com/zonghaoyuan/infiplot/blob/staging/CLA.md).
+
+> If you have not yet signed, read the [CLA](https://github.com/zonghaoyuan/infiplot/blob/staging/CLA.md)
+> and reply to this PR with: `I have read the CLA Document and I hereby sign the CLA`
+> You only need to sign once.
+
+## Self-check
+
+- [ ] I have read [`CONTRIBUTING.md`](../CONTRIBUTING.md)
+- [ ] `pnpm typecheck` passes
+- [ ] `pnpm lint` passes
+
+## Type of change
+
+<!-- Check all that apply -->
+
+- [ ] `feat` — new feature
+- [ ] `fix` — bug fix
+- [ ] `perf` / `refactor` — performance or code-quality change
+- [ ] `docs` — documentation only
+- [ ] `chore` — build, deps, tooling, etc.