Add Spanish report translation

[adefang512-cmyk]

Maintainer need

OSS Careboard currently renders English and Chinese reports. A Spanish translation would make the read-only briefing useful to more open-source teams.

Suggested scope

• Add es to the CLI language choices.
• Translate all report and briefing strings in src/oss_careboard/report.py.
• Add focused tests for Spanish headings, coverage warnings, and the human-review notice.
• Update the README language reference.

Acceptance criteria

• Existing English and Chinese output remains unchanged.
• oss-careboard --language es renders a complete report.
• The full unit test suite passes.

Please avoid machine-translated text without a fluent human review.