chore: add CODEOWNERS so reviews are routed automatically

[ubugeeei]

Problem

`.github/CODEOWNERS` is missing. As contributors join, reviewers must be manually requested on every PR.

Expected

• A CODEOWNERS file that defaults all paths to `@ubugeeei` for now.
• Carve-outs ready for future expansion (`backend/`, `render/`, `docs/`).

Acceptance

• File at `.github/CODEOWNERS`.
• GitHub UI shows the expected default reviewer on new PRs.