Gitea second-factor bypass through OpenID identity linking

Gitea's OpenID connect handler accepts a user's password without enforcing enrolled TOTP or WebAuthn verification before completing sign-in and linking an OpenID identifier. When OpenID sign-in is enabled, an attacker who knows the victim's password can obtain a session and link an attacker-controlled identifier. The attacker can then sign in through that identifier without the victim's password or second factor, and a password change does not remove the link.