[Chore] Add .github/CODEOWNERS for review routing

[grantfox-oss]

Telegram (ask questions / claim the issue here first): https://t.me/+DOylgFv1jyJlNzM0

Why this matters

There is no CODEOWNERS file anywhere in the repo (.github/ has ISSUE_TEMPLATE, workflows, dependabot.yml, pull_request_template.md, codecov.yml but no CODEOWNERS), so PRs touching backend/, frontend/, or contracts/ have no auto-assigned reviewers in this monorepo.

Acceptance criteria

• Add .github/CODEOWNERS mapping backend/, frontend/, contracts/, docs/, and .github/ to appropriate maintainers
• Confirm GitHub recognizes the file (no parse errors in the Code owners UI)

Files to touch

• .github/CODEOWNERS

Out of scope

• Configuring branch protection / required reviews