Rate-limit authentication endpoints

Follow-up slice of #227.

Apply the established API rate-limit policy to the authentication router, which is outside the existing /api limiter.

Scope:
- protect authentication code exchange and logout routes consistently;
- add a focused regression test for requests exceeding the configured IP window;
- retain the current response contract and CSRF protection;
- do not add, inspect, store, or search email data.

Acceptance criteria:
- repeated authentication-route requests receive the standard 429 response;
- normal sign-in and logout behavior still works within the limit;
- server tests and CodeQL pass.

Production rollout validation remains part of #176.

Start by reading the established API rate-limit policy from follow-up issue #227 and locating the authentication router, especially the code exchange and logout routes. Add focused server-test coverage for requests exceeding the configured IP window, then verify the standard 429 response, existing sign-in/logout behavior, CSRF protection, and CodeQL results without changing the response contract.

Vom Indexierungsmodell aus dem Issue-Text verfasst.