Add BSD-3-Clause license

[caddoo]

Why

• Establish a clear, explicit BSD-3-Clause license for the project under Matomo Open Source Analytics, enabling reuse with defined terms and attribution.

What

• Added LICENSE containing the BSD-3-Clause text and 2026 copyright.
• Added "license": "BSD-3-Clause" to package.json for tooling and registry metadata.

Why third-party notices were left out

• The repo does not ship bundled dependencies (dist artifacts are ignored), so distributing third-party license notices here isn’t required. If a bundled artifact is ever distributed externally, we should include third-party notices alongside that release package.

Checklist

• [✔] I have understood, reviewed, and tested all AI outputs before use
• [✔] All AI instructions respect security, IP, and privacy rules