GHSA-8m7c-hf24-5g47

### Summary Two concurrent token-exchange requests using the same OAuth authorization code could each mint a distinct valid `(access_token, refresh_token)` pair, breaking the single-use guarantee that PKCE relies on.

### Details The token-exchange flow read `is_used` and called `markAsUsed` as an unconditional update at the end of the path. A new `OAuthAuthorizationCode.claimByCode` method now performs an atomic compare-and-swap (`WHERE code = ? AND is_used = false`) and is called immediately before `OAuthToken.insert`, after redirect-URI, PKCE, and client authentication have all succeeded. Only the first concurrent caller's `UPDATE` wins; the rest see `invalid_grant: Authorization code has already been used`.

### Impact An attacker who has observed an authorization code and the corresponding PKCE verifier (for example through a malicious OAuth-aware client or by racing a real exchange) could obtain a long-lived refresh token in addition to the legitimate one.

### Credit This issue was reported by [@eddieran](https://github.com/eddieran).