Invite a GitHub user to the account

Owner-only. github_username is validated locally (1–39 bytes, ^[A-Za-z0-9]+(-[A-Za-z0-9]+)*$) before any outbound call — 400 invalid_github_username on failure. A valid login is resolved via GitHub's public, unauthenticated GET /users/{login}: 404 github_user_not_found if GitHub reports no such user, 502 github_error on any other GitHub failure. 409 member_exists if a user with that GitHub ID is already a member; 409 invite_exists if a live invite for them already exists. Invites expire after 7 days.