chore: add CODEOWNERS

[rennf93]

Adds a root CODEOWNERS file so the code-owner review requirement in the master ruleset resolves against an actual owner.

[github-actions]

Thanks for opening your first pull request on flaskapi-guard!

Quick checklist before review (most of these are enforced by CI, but worth a glance):

• make lint — ruff format, ruff check, mypy, and vulture
• make test (or make local-test) — full test suite passes with 100% coverage
• make security — bandit, safety, and pip-audit all clean
• make quality — radon and xenon complexity thresholds respected
• No # noqa / # type: ignore shortcuts; pre-existing violations in touched files are fixed
• Added an entry under ## Unreleased in CHANGELOG.md
• If you touched a public API, updated the relevant page under docs/
• If the change involves engine logic, the corresponding fix landed in guard-core first

See CONTRIBUTING.md for the full workflow and the Code of Conduct for the community standards we follow.

Welcome aboard — a maintainer will review shortly.