feat(frontend-auth): add NextAuth.js v5 Keycloak integration (partial - Task 10)

- Install next-auth@5.0.0-beta.30 and @auth/core@0.34.3
- Configure Keycloak OIDC provider with JWT and session callbacks
- Add module augmentation for JWT and Session types (clubs claim support)
- Export auth handlers and configuration

INCOMPLETE: Missing middleware.ts, useActiveClub() hook, API utility, and tests
Will complete in follow-up session resumption

frontend/src/auth/index.ts

@@ -0,0 +1 @@
+export { auth, signIn, signOut, handlers } from "./auth"