Rate-limit authentication endpoints

[coder13]

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.