docs: update CONTRIBUTING.md to include maintainers and approval requirements; modify CODEOWNERS for broader review responsibility

Author: robinebers

.github/CODEOWNERS

@@ -1,2 +1,2 @@
-# Default: Robin reviews everything
-* @robinebers
+# Default: maintainers review everything
+* @robinebers @validatedev @davidarny

CONTRIBUTING.md

@@ -77,6 +77,14 @@ Don't open a PR for large features without discussing first. [Open an issue](htt
 - Follow existing patterns in the codebase
 - No new dependencies without justification
 
+## Maintainers
+
+- [@robinebers](https://github.com/robinebers) (lead)
+- [@validatedev](https://github.com/validatedev)
+- [@davidarny](https://github.com/davidarny)
+
+All PRs require approval from at least 2 maintainers before merging.
+
 ## Questions?
 
 Open a [bug report](https://github.com/robinebers/openusage/issues/new?template=bug_report.yml) or [feature request](https://github.com/robinebers/openusage/issues/new?template=feature_request.yml) using the issue templates.