Gitea API token scope escalation through HTTP signatures

Gitea allows a token with write:user scope to register an attacker-controlled SSH public key on the token owner's account. API requests signed with the corresponding private key authenticate as that account through HTTP Signature without API token scope information, causing route-level token scope checks to be skipped. This converts a user-scoped token into an unscoped API authentication method while retaining the account's normal role and repository permissions.